Fix figuring out how to get the root path on Windows.
Fix figuring out how to get the root path on Windows.

Summary:
The FileHashCaches are set up to resolve paths outside of the
default root directory. On Windows, there may be more than
one root filesystem, and this isn't denoted with "/".

Test Plan: `buck build buck-win` on Windows works

vschs007/buck,LegNeato/buck,SeleniumHQ/buck,bocon13/buck,rowillia/buck,vschs007/buck,JoelMarcey/buck,dsyang/buck,vine/buck,zhan-xiong/buck,raviagarwal7/buck,marcinkwiatkowski/buck,illicitonion/buck,bocon13/buck,clonetwin26/buck,grumpyjames/buck,sdwilsh/buck,rowillia/buck,daedric/buck,zhan-xiong/buck,tgummerer/buck,dsyang/buck,marcinkwiatkowski/buck,liuyang-li/buck,dsyang/buck,OkBuilds/buck,davido/buck,robbertvanginkel/buck,rhencke/buck,robbertvanginkel/buck,clonetwin26/buck,vine/buck,romanoid/buck,shs96c/buck,SeleniumHQ/buck,Addepar/buck,clonetwin26/buck,tgummerer/buck,dsyang/buck,JoelMarcey/buck,brettwooldridge/buck,daedric/buck,bocon13/buck,SeleniumHQ/buck,Addepar/buck,nguyentruongtho/buck,SeleniumHQ/buck,marcinkwiatkowski/buck,daedric/buck,janicduplessis/buck,justinmuller/buck,grumpyjames/buck,shs96c/buck,rhencke/buck,davido/buck,shs96c/buck,romanoid/buck,sdwilsh/buck,bocon13/buck,nguyentruongtho/buck,rmaz/buck,sdwilsh/buck,justinmuller/buck,sdwilsh/buck,Addepar/buck,shybovycha/buck,sdwilsh/buck,mikekap/buck,Distrotech/buck,darkforestzero/buck,Addepar/buck,mikekap/buck,Dominator008/buck,Distrotech/buck,grumpyjames/buck,SeleniumHQ/buck,rowillia/buck,mogers/buck,davido/buck,illicitonion/buck,JoelMarcey/buck,justinmuller/buck,ilya-klyuchnikov/buck,darkforestzero/buck,JoelMarcey/buck,shybovycha/buck,darkforestzero/buck,raviagarwal7/buck,Addepar/buck,ilya-klyuchnikov/buck,robbertvanginkel/buck,vschs007/buck,bocon13/buck,facebook/buck,OkBuilds/buck,janicduplessis/buck,vschs007/buck,rhencke/buck,clonetwin26/buck,clonetwin26/buck,darkforestzero/buck,LegNeato/buck,illicitonion/buck,robbertvanginkel/buck,mogers/buck,marcinkwiatkowski/buck,vine/buck,mikekap/buck,raviagarwal7/buck,rhencke/buck,LegNeato/buck,Addepar/buck,marcinkwiatkowski/buck,vschs007/buck,daedric/buck,vschs007/buck,zhan-xiong/buck,LegNeato/buck,LegNeato/buck,daedric/buck,illicitonion/buck,illicitonion/buck,rowillia/buck,LegNeato/buck,raviagarwal7/buck,daedric/buck,mikekap/buck,liuyang-li/buck,JoelMarcey/buck,vine/buck,LegNeato/buck,rmaz/buck,tgummerer/buck,mikekap/buck,vschs007/buck,mikekap/buck,zpao/buck,JoelMarcey/buck,romanoid/buck,clonetwin26/buck,vine/buck,shs96c/buck,liuyang-li/buck,dsyang/buck,k21/buck,mikekap/buck,rhencke/buck,Distrotech/buck,shybovycha/buck,robbertvanginkel/buck,OkBuilds/buck,marcinkwiatkowski/buck,Dominator008/buck,k21/buck,darkforestzero/buck,shybovycha/buck,raviagarwal7/buck,raviagarwal7/buck,rowillia/buck,raviagarwal7/buck,romanoid/buck,marcinkwiatkowski/buck,brettwooldridge/buck,rmaz/buck,rmaz/buck,marcinkwiatkowski/buck,SeleniumHQ/buck,illicitonion/buck,Distrotech/buck,romanoid/buck,vschs007/buck,bocon13/buck,zhan-xiong/buck,daedric/buck,Dominator008/buck,sdwilsh/buck,facebook/buck,Dominator008/buck,ilya-klyuchnikov/buck,clonetwin26/buck,marcinkwiatkowski/buck,brettwooldridge/buck,dsyang/buck,Distrotech/buck,mogers/buck,brettwooldridge/buck,grumpyjames/buck,davido/buck,illicitonion/buck,vschs007/buck,shybovycha/buck,davido/buck,Distrotech/buck,davido/buck,shs96c/buck,k21/buck,JoelMarcey/buck,OkBuilds/buck,Dominator008/buck,SeleniumHQ/buck,k21/buck,justinmuller/buck,Distrotech/buck,robbertvanginkel/buck,OkBuilds/buck,illicitonion/buck,mikekap/buck,tgummerer/buck,bocon13/buck,rmaz/buck,OkBuilds/buck,justinmuller/buck,nguyentruongtho/buck,k21/buck,shs96c/buck,romanoid/buck,JoelMarcey/buck,liuyang-li/buck,mogers/buck,JoelMarcey/buck,zpao/buck,mikekap/buck,ilya-klyuchnikov/buck,daedric/buck,liuyang-li/buck,Dominator008/buck,k21/buck,rmaz/buck,raviagarwal7/buck,justinmuller/buck,rmaz/buck,darkforestzero/buck,rmaz/buck,mikekap/buck,darkforestzero/buck,SeleniumHQ/buck,Distrotech/buck,mogers/buck,liuyang-li/buck,brettwooldridge/buck,kageiit/buck,LegNeato/buck,bocon13/buck,grumpyjames/buck,clonetwin26/buck,kageiit/buck,tgummerer/buck,darkforestzero/buck,davido/buck,grumpyjames/buck,rowillia/buck,SeleniumHQ/buck,LegNeato/buck,LegNeato/buck,raviagarwal7/buck,kageiit/buck,liuyang-li/buck,Distrotech/buck,illicitonion/buck,zpao/buck,ilya-klyuchnikov/buck,darkforestzero/buck,zhan-xiong/buck,mogers/buck,mikekap/buck,clonetwin26/buck,bocon13/buck,dsyang/buck,Dominator008/buck,liuyang-li/buck,ilya-klyuchnikov/buck,dsyang/buck,grumpyjames/buck,Addepar/buck,shs96c/buck,bocon13/buck,shs96c/buck,Dominator008/buck,rmaz/buck,clonetwin26/buck,zhan-xiong/buck,LegNeato/buck,romanoid/buck,daedric/buck,marcinkwiatkowski/buck,Addepar/buck,k21/buck,robbertvanginkel/buck,facebook/buck,facebook/buck,kageiit/buck,JoelMarcey/buck,zhan-xiong/buck,janicduplessis/buck,vine/buck,shybovycha/buck,JoelMarcey/buck,rowillia/buck,tgummerer/buck,mogers/buck,rhencke/buck,justinmuller/buck,zhan-xiong/buck,bocon13/buck,sdwilsh/buck,grumpyjames/buck,SeleniumHQ/buck,sdwilsh/buck,raviagarwal7/buck,mogers/buck,nguyentruongtho/buck,Dominator008/buck,vine/buck,brettwooldridge/buck,shybovycha/buck,shybovycha/buck,facebook/buck,illicitonion/buck,robbertvanginkel/buck,liuyang-li/buck,janicduplessis/buck,brettwooldridge/buck,davido/buck,Distrotech/buck,justinmuller/buck,shybovycha/buck,zhan-xiong/buck,rhencke/buck,nguyentruongtho/buck,vschs007/buck,k21/buck,ilya-klyuchnikov/buck,vschs007/buck,Distrotech/buck,liuyang-li/buck,raviagarwal7/buck,clonetwin26/buck,illicitonion/buck,shs96c/buck,darkforestzero/buck,Addepar/buck,justinmuller/buck,dsyang/buck,tgummerer/buck,Addepar/buck,sdwilsh/buck,shs96c/buck,shybovycha/buck,mogers/buck,raviagarwal7/buck,nguyentruongtho/buck,robbertvanginkel/buck,illicitonion/buck,vschs007/buck,k21/buck,robbertvanginkel/buck,shybovycha/buck,darkforestzero/buck,raviagarwal7/buck,janicduplessis/buck,tgummerer/buck,kageiit/buck,brettwooldridge/buck,daedric/buck,liuyang-li/buck,grumpyjames/buck,tgummerer/buck,bocon13/buck,OkBuilds/buck,vine/buck,justinmuller/buck,romanoid/buck,ilya-klyuchnikov/buck,rmaz/buck,marcinkwiatkowski/buck,janicduplessis/buck,sdwilsh/buck,nguyentruongtho/buck,SeleniumHQ/buck,zpao/buck,ilya-klyuchnikov/buck,romanoid/buck,dsyang/buck,Addepar/buck,kageiit/buck,grumpyjames/buck,rhencke/buck,rhencke/buck,rmaz/buck,grumpyjames/buck,k21/buck,zpao/buck,shs96c/buck,vine/buck,rmaz/buck,janicduplessis/buck,shs96c/buck,zpao/buck,romanoid/buck,ilya-klyuchnikov/buck,JoelMarcey/buck,davido/buck,rowillia/buck,brettwooldridge/buck,dsyang/buck,robbertvanginkel/buck,janicduplessis/buck,davido/buck,vine/buck,davido/buck,janicduplessis/buck,davido/buck,ilya-klyuchnikov/buck,vine/buck,romanoid/buck,k21/buck,sdwilsh/buck,facebook/buck,sdwilsh/buck,rowillia/buck,zpao/buck,brettwooldridge/buck,janicduplessis/buck,davido/buck,Addepar/buck,SeleniumHQ/buck,mogers/buck,SeleniumHQ/buck,LegNeato/buck,k21/buck,Dominator008/buck,clonetwin26/buck,tgummerer/buck,shybovycha/buck,justinmuller/buck,LegNeato/buck,zhan-xiong/buck,OkBuilds/buck,daedric/buck,marcinkwiatkowski/buck,tgummerer/buck,janicduplessis/buck,clonetwin26/buck,zhan-xiong/buck,OkBuilds/buck,justinmuller/buck,brettwooldridge/buck,robbertvanginkel/buck,sdwilsh/buck,JoelMarcey/buck,illicitonion/buck,rhencke/buck,daedric/buck,rmaz/buck,OkBuilds/buck,brettwooldridge/buck,darkforestzero/buck,robbertvanginkel/buck,janicduplessis/buck,OkBuilds/buck,mikekap/buck,romanoid/buck,shs96c/buck,rowillia/buck,ilya-klyuchnikov/buck,grumpyjames/buck,Dominator008/buck,kageiit/buck,brettwooldridge/buck,OkBuilds/buck,facebook/buck,dsyang/buck,k21/buck,daedric/buck,rowillia/buck,romanoid/buck,justinmuller/buck,OkBuilds/buck,shybovycha/buck,vschs007/buck,rhencke/buck,zhan-xiong/buck,zhan-xiong/buck,Dominator008/buck,rowillia/buck,mogers/buck,Addepar/buck,darkforestzero/buck,marcinkwiatkowski/buck,ilya-klyuchnikov/buck,dsyang/buck