Add support for tab completion in bash.
Add support for tab completion in bash.

Summary:
From https://github.com/facebook/buck/pull/54.

When the bash_completion script is loaded, the user
can use the tab key to automatically complete the name of the
`buck command`, or if there are multiple commands beginning with
the same letter(s) suggest commands to use.

Test Plan:
Install the completion script:

  $ ./scripts/bash_completion

Suggestion of all available commands:

  $ buck
  audit build cache clean install project targets test uninstall

Suggestion of available commands with given starting letter:

  $ buck c
  cache clean

Completion of command with given starting letter

  $ buck b

("b" is auto-completed to "build")

Learn-Android-app/buck,rhencke/buck,Distrotech/buck,GerritCodeReview/buck,1yvT0s/buck,ilya-klyuchnikov/buck,pwz3n0/buck,Dominator008/buck,facebook/buck,dsyang/buck,darkforestzero/buck,tgummerer/buck,SeleniumHQ/buck,zpao/buck,liuyang-li/buck,vschs007/buck,shs96c/buck,bocon13/buck,Addepar/buck,dpursehouse/buck,zhuxiaohao/buck,lukw00/buck,MarkRunWu/buck,vschs007/buck,bocon13/buck,rhencke/buck,clonetwin26/buck,k21/buck,davido/buck,OkBuilds/buck,daedric/buck,illicitonion/buck,daedric/buck,denizt/buck,saleeh93/buck-cutom,rowillia/buck,Heart2009/buck,Addepar/buck,stuhood/buck,justinmuller/buck,brettwooldridge/buck,neonichu/buck,shybovycha/buck,grumpyjames/buck,dsyang/buck,janicduplessis/buck,vine/buck,Dominator008/buck,Distrotech/buck,OkBuilds/buck,vine/buck,brettwooldridge/buck,jackminicloud/buck,zhan-xiong/buck,ilya-klyuchnikov/buck,rmaz/buck,MarkRunWu/buck,saleeh93/buck-cutom,daedric/buck,rowillia/buck,MarkRunWu/buck,hgl888/buck,Addepar/buck,JoelMarcey/buck,neonichu/buck,davido/buck,illicitonion/buck,saleeh93/buck-cutom,sdwilsh/buck,pwz3n0/buck,jackminicloud/buck,raviagarwal7/buck,1yvT0s/buck,denizt/buck,mikekap/buck,davido/buck,illicitonion/buck,mread/buck,ilya-klyuchnikov/buck,darkforestzero/buck,kageiit/buck,daedric/buck,darkforestzero/buck,zpao/buck,daedric/buck,janicduplessis/buck,mnuessler/buck,grumpyjames/buck,bocon13/buck,robbertvanginkel/buck,Heart2009/buck,marcinkwiatkowski/buck,tgummerer/buck,davido/buck,sdwilsh/buck,neonichu/buck,jackminicloud/buck,daedric/buck,Dominator008/buck,zpao/buck,zhuxiaohao/buck,mogers/buck,mogers/buck,SeleniumHQ/buck,vschs007/buck,grumpyjames/buck,raviagarwal7/buck,liuyang-li/buck,shybovycha/buck,vine/buck,mnuessler/buck,vine/buck,thinkernel/buck,mogers/buck,grumpyjames/buck,facebook/buck,brettwooldridge/buck,raviagarwal7/buck,shybovycha/buck,mnuessler/buck,grumpyjames/buck,rhencke/buck,vschs007/buck,stuhood/buck,rowillia/buck,vschs007/buck,Distrotech/buck,brettwooldridge/buck,davido/buck,romanoid/buck,facebook/buck,Heart2009/buck,tgummerer/buck,dpursehouse/buck,marcinkwiatkowski/buck,dpursehouse/buck,neonichu/buck,Addepar/buck,LegNeato/buck,Learn-Android-app/buck,Learn-Android-app/buck,nguyentruongtho/buck,zhan-xiong/buck,Learn-Android-app/buck,bocon13/buck,dpursehouse/buck,Dominator008/buck,dsyang/buck,justinmuller/buck,romanoid/buck,Addepar/buck,denizt/buck,luiseduardohdbackup/buck,siddhartharay007/buck,brettwooldridge/buck,Heart2009/buck,1yvT0s/buck,pwz3n0/buck,raviagarwal7/buck,dsyang/buck,romanoid/buck,OkBuilds/buck,clonetwin26/buck,sdwilsh/buck,MarkRunWu/buck,OkBuilds/buck,brettwooldridge/buck,Addepar/buck,romanoid/buck,JoelMarcey/buck,davido/buck,shs96c/buck,dushmis/buck,bocon13/buck,rmaz/buck,liuyang-li/buck,romanoid/buck,artiya4u/buck,Learn-Android-app/buck,stuhood/buck,zhuxiaohao/buck,marcinkwiatkowski/buck,darkforestzero/buck,artiya4u/buck,robbertvanginkel/buck,ilya-klyuchnikov/buck,pwz3n0/buck,mread/buck,zhan-xiong/buck,daedric/buck,OkBuilds/buck,ilya-klyuchnikov/buck,raviagarwal7/buck,Distrotech/buck,darkforestzero/buck,rmaz/buck,luiseduardohdbackup/buck,pwz3n0/buck,mread/buck,hgl888/buck,shs96c/buck,robbertvanginkel/buck,shs96c/buck,dushmis/buck,facebook/buck,vschs007/buck,JoelMarcey/buck,SeleniumHQ/buck,SeleniumHQ/buck,neonichu/buck,mnuessler/buck,mikekap/buck,dpursehouse/buck,romanoid/buck,justinmuller/buck,davido/buck,raviagarwal7/buck,dsyang/buck,k21/buck,zhuxiaohao/buck,davido/buck,rmaz/buck,raviagarwal7/buck,stuhood/buck,robbertvanginkel/buck,raviagarwal7/buck,zhuxiaohao/buck,janicduplessis/buck,stuhood/buck,Dominator008/buck,mikekap/buck,pwz3n0/buck,robbertvanginkel/buck,tgummerer/buck,grumpyjames/buck,illicitonion/buck,OkBuilds/buck,Dominator008/buck,MarkRunWu/buck,justinmuller/buck,neonichu/buck,LegNeato/buck,vschs007/buck,siddhartharay007/buck,justinmuller/buck,brettwooldridge/buck,rmaz/buck,Addepar/buck,zhan-xiong/buck,MarkRunWu/buck,Dominator008/buck,stuhood/buck,OkBuilds/buck,shybovycha/buck,k21/buck,lukw00/buck,darkforestzero/buck,luiseduardohdbackup/buck,mikekap/buck,LegNeato/buck,liuyang-li/buck,mogers/buck,dushmis/buck,shybovycha/buck,siddhartharay007/buck,1yvT0s/buck,dsyang/buck,vschs007/buck,dushmis/buck,rmaz/buck,daedric/buck,liuyang-li/buck,darkforestzero/buck,lukw00/buck,k21/buck,davido/buck,Heart2009/buck,JoelMarcey/buck,dsyang/buck,Distrotech/buck,stuhood/buck,1yvT0s/buck,belomx/open_tools,romanoid/buck,ilya-klyuchnikov/buck,shybovycha/buck,OkBuilds/buck,saleeh93/buck-cutom,marcinkwiatkowski/buck,vine/buck,darkforestzero/buck,raviagarwal7/buck,rhencke/buck,rmaz/buck,davido/buck,Addepar/buck,mogers/buck,hgl888/buck,justinmuller/buck,sdwilsh/buck,JoelMarcey/buck,neonichu/buck,clonetwin26/buck,pwz3n0/buck,rowillia/buck,darkforestzero/buck,nguyentruongtho/buck,zpao/buck,jackminicloud/buck,daedric/buck,nguyentruongtho/buck,justinmuller/buck,facebook/buck,SeleniumHQ/buck,shs96c/buck,Distrotech/buck,liuyang-li/buck,rmaz/buck,mogers/buck,dpursehouse/buck,sdwilsh/buck,sdwilsh/buck,luiseduardohdbackup/buck,vine/buck,clonetwin26/buck,romanoid/buck,kageiit/buck,illicitonion/buck,grumpyjames/buck,facebook/buck,hgl888/buck,sdwilsh/buck,JoelMarcey/buck,marcinkwiatkowski/buck,JoelMarcey/buck,janicduplessis/buck,mread/buck,zhan-xiong/buck,1yvT0s/buck,GerritCodeReview/buck,stuhood/buck,k21/buck,dushmis/buck,siddhartharay007/buck,stuhood/buck,mogers/buck,bocon13/buck,rowillia/buck,clonetwin26/buck,lukw00/buck,vschs007/buck,dsyang/buck,LegNeato/buck,luiseduardohdbackup/buck,thinkernel/buck,mogers/buck,illicitonion/buck,artiya4u/buck,illicitonion/buck,Learn-Android-app/buck,kageiit/buck,janicduplessis/buck,SeleniumHQ/buck,rmaz/buck,LegNeato/buck,liuyang-li/buck,ilya-klyuchnikov/buck,Heart2009/buck,kageiit/buck,artiya4u/buck,Dominator008/buck,marcinkwiatkowski/buck,luiseduardohdbackup/buck,LegNeato/buck,romanoid/buck,Addepar/buck,justinmuller/buck,zhan-xiong/buck,shs96c/buck,mikekap/buck,raviagarwal7/buck,luiseduardohdbackup/buck,dpursehouse/buck,grumpyjames/buck,brettwooldridge/buck,janicduplessis/buck,Dominator008/buck,neonichu/buck,JoelMarcey/buck,Learn-Android-app/buck,dpursehouse/buck,marcinkwiatkowski/buck,sdwilsh/buck,SeleniumHQ/buck,Dominator008/buck,robbertvanginkel/buck,janicduplessis/buck,artiya4u/buck,k21/buck,lukw00/buck,robbertvanginkel/buck,rhencke/buck,Distrotech/buck,siddhartharay007/buck,rhencke/buck,janicduplessis/buck,marcinkwiatkowski/buck,rhencke/buck,liuyang-li/buck,k21/buck,rhencke/buck,nguyentruongtho/buck,zhan-xiong/buck,JoelMarcey/buck,vine/buck,dsyang/buck,vschs007/buck,stuhood/buck,Addepar/buck,dsyang/buck,zpao/buck,pwz3n0/buck,OkBuilds/buck,brettwooldridge/buck,nguyentruongtho/buck,vine/buck,tgummerer/buck,clonetwin26/buck,ilya-klyuchnikov/buck,SeleniumHQ/buck,neonichu/buck,facebook/buck,dsyang/buck,illicitonion/buck,shs96c/buck,shs96c/buck,grumpyjames/buck,vine/buck,SeleniumHQ/buck,clonetwin26/buck,zhan-xiong/buck,hgl888/buck,zhan-xiong/buck,1yvT0s/buck,SeleniumHQ/buck,brettwooldridge/buck,LegNeato/buck,zhuxiaohao/buck,Learn-Android-app/buck,mread/buck,illicitonion/buck,illicitonion/buck,rowillia/buck,marcinkwiatkowski/buck,kageiit/buck,MarkRunWu/buck,ilya-klyuchnikov/buck,daedric/buck,Distrotech/buck,Distrotech/buck,hgl888/buck,clonetwin26/buck,zhuxiaohao/buck,dushmis/buck,mikekap/buck,kageiit/buck,mnuessler/buck,dushmis/buck,LegNeato/buck,brettwooldridge/buck,ilya-klyuchnikov/buck,bocon13/buck,mikekap/buck,OkBuilds/buck,tgummerer/buck,bocon13/buck,mikekap/buck,tgummerer/buck,shybovycha/buck,OkBuilds/buck,mogers/buck,romanoid/buck,justinmuller/buck,robbertvanginkel/buck,illicitonion/buck,rmaz/buck,darkforestzero/buck,pwz3n0/buck,justinmuller/buck,marcinkwiatkowski/buck,romanoid/buck,brettwooldridge/buck,davido/buck,rhencke/buck,Dominator008/buck,daedric/buck,GerritCodeReview/buck,vine/buck,JoelMarcey/buck,bocon13/buck,GerritCodeReview/buck,SeleniumHQ/buck,robbertvanginkel/buck,dushmis/buck,luiseduardohdbackup/buck,k21/buck,zhuxiaohao/buck,darkforestzero/buck,zhan-xiong/buck,1yvT0s/buck,darkforestzero/buck,shybovycha/buck,artiya4u/buck,Heart2009/buck,siddhartharay007/buck,nguyentruongtho/buck,lukw00/buck,Dominator008/buck,dushmis/buck,pwz3n0/buck,janicduplessis/buck,liuyang-li/buck,zhan-xiong/buck,lukw00/buck,janicduplessis/buck,janicduplessis/buck,shs96c/buck,justinmuller/buck,darkforestzero/buck,daedric/buck,clonetwin26/buck,shybovycha/buck,rowillia/buck,JoelMarcey/buck,vine/buck,LegNeato/buck,sdwilsh/buck,belomx/open_tools,saleeh93/buck-cutom,mikekap/buck,Learn-Android-app/buck,Heart2009/buck,shs96c/buck,JoelMarcey/buck,mikekap/buck,dsyang/buck,ilya-klyuchnikov/buck,siddhartharay007/buck,lukw00/buck,vschs007/buck,SeleniumHQ/buck,zhan-xiong/buck,LegNeato/buck,justinmuller/buck,shs96c/buck,saleeh93/buck-cutom,daedric/buck,pwz3n0/buck,denizt/buck,hgl888/buck,shybovycha/buck,liuyang-li/buck,mnuessler/buck,raviagarwal7/buck,shybovycha/buck,robbertvanginkel/buck,janicduplessis/buck,ilya-klyuchnikov/buck,ilya-klyuchnikov/buck,romanoid/buck,marcinkwiatkowski/buck,rowillia/buck,Distrotech/buck,artiya4u/buck,vschs007/buck,mread/buck,tgummerer/buck,clonetwin26/buck,sdwilsh/buck,siddhartharay007/buck,LegNeato/buck,mnuessler/buck,mogers/buck,bocon13/buck,belomx/open_tools,mnuessler/buck,davido/buck,rhencke/buck,illicitonion/buck,rowillia/buck,luiseduardohdbackup/buck,grumpyjames/buck,artiya4u/buck,OkBuilds/buck,bocon13/buck,vschs007/buck,mikekap/buck,k21/buck,MarkRunWu/buck,rhencke/buck,liuyang-li/buck,rmaz/buck,sdwilsh/buck,bocon13/buck,zpao/buck,stuhood/buck,hgl888/buck,mikekap/buck,robbertvanginkel/buck,k21/buck,rowillia/buck,shs96c/buck,denizt/buck,raviagarwal7/buck,Learn-Android-app/buck,artiya4u/buck,zpao/buck,siddhartharay007/buck,tgummerer/buck,OkBuilds/buck,tgummerer/buck,artiya4u/buck,sdwilsh/buck,Addepar/buck,clonetwin26/buck,illicitonion/buck,justinmuller/buck,dsyang/buck,Addepar/buck,LegNeato/buck,mogers/buck,shybovycha/buck,LegNeato/buck,k21/buck,shybovycha/buck,robbertvanginkel/buck,kageiit/buck,Addepar/buck,Distrotech/buck,k21/buck,SeleniumHQ/buck,lukw00/buck,raviagarwal7/buck,Learn-Android-app/buck,davido/buck,rmaz/buck,hgl888/buck,tgummerer/buck,robbertvanginkel/buck,marcinkwiatkowski/buck,thinkernel/buck,zhuxiaohao/buck,sdwilsh/buck,rmaz/buck,thinkernel/buck,romanoid/buck,lukw00/buck,Heart2009/buck,marcinkwiatkowski/buck,zhan-xiong/buck,grumpyjames/buck,brettwooldridge/buck,clonetwin26/buck,1yvT0s/buck,nguyentruongtho/buck,belomx/open_tools,dushmis/buck,rowillia/buck,rowillia/buck,shs96c/buck,mnuessler/buck,artiya4u/buck,JoelMarcey/buck,clonetwin26/buck,grumpyjames/buck,lukw00/buck,k21/buck