Speed up rehash process when there are many Ruby versions
Speed up rehash process when there are many Ruby versions

On my system that has 25 versions under rbenv, this speeds up rehash
almost 3-fold:

- before: 391 ms
- after:  134 ms

This is achieved by removing duplicate names of executables before
registering them as shims. Since most Rubies will share a lot of the
same executable names ("ruby", "rake", "bundle", ...), this is a
considerable reduction in number of shims registered.

docwhat/rbenv,beck/rbenv,viclim/rbenv,lozybean/rbenv,cehoffman/luaenv,jmatbastos/rbenv,RinSer/rbenv,peterpih/rbenv,princeofdarkness76/rbenv,hlecuanda/pyenv,mediadoneright/rbenv,avdhoweb/rbenv,cehoffman/luaenv,codesdk/rbenv,VShangxiao/pyenv,janusnic/rbenv,BanzaiMan/pyenv,nodenv/nodenv,cyril/rbenv,dark5un/nodenv,josegonzalez/rbenv,intfrr/rbenv,codesdk/rbenv,stivenrgalvis/rbenv,rridhan/rbenv,jeongdeoksu/rbenv,lozybean/pyenv,pyenv/pyenv,janusnic/rbenv,rbenv/rbenv,SamuraiT/rbenv,obimod/pyenv,jmatbastos/rbenv,hlecuanda/pyenv,howtolearntocode/rbenv,adarshmuthappa/nodenv,jeremy/rbenv,BanzaiMan/pyenv,peterpih/rbenv,jasonkarns/rbenv,bradynpoulsen/rbenv,jmatbastos/rbenv,princeofdarkness76/rbenv,nodenv/nodenv,genome-vendor/pyenv,michael-gabenna/rbenv,yyuu/pyenv,howtolearntocode/rbenv,viclim/rbenv,jasonkarns/rbenv,recruit-mtl/pyenv,yeonsh/pyenv,jeongdeoksu/rbenv,viclim/rbenv,docwhat/rbenv,mediadoneright/rbenv,zmwangx/pyenv,yyuu/pyenv,lozybean/rbenv,rridhan/rbenv,yogeshkakde/rbenv,jmatbastos/rbenv,jeongdeoksu/rbenv,habinmusic/pyenv,xshwy/pyenv,Angeldude/rbenv,mediadoneright/rbenv,pumpkineater/rbenv,jeremy/rbenv,Ferocia/rbenv,intfrr/rbenv,zmwangx/pyenv,lozybean/rbenv,avdhoweb/rbenv,recruit-mtl/pyenv,cehoffman/rbenv,docwhat/rbenv,yeonsh/pyenv,yyyc514/rbenv,peterpih/rbenv,sstephenson/rbenv,reyesyang/rbenv,yogeshkakde/rbenv,jeremy/rbenv,pumpkineater/rbenv,RinSer/rbenv,beck/rbenv,jrolfs/nodenv,sstephenson/rbenv,cehoffman/rbenv,yyyc514/rbenv,genome-vendor/pyenv,yogeshkakde/rbenv,sstephenson/rbenv,Ferocia/rbenv,avdhoweb/rbenv,stivenrgalvis/rbenv,obimod/pyenv,rbenv/rbenv,rridhan/rbenv,cehoffman/pyenv,cehoffman/nodenv,syndbg/goenv,takluyver/pyenv,fly19890211/pyenv,xshwy/pyenv,fly19890211/pyenv,jmatbastos/rbenv,UCASREN/pyenv,OiNutter/nodenv,pyenv/pyenv,VShangxiao/pyenv,codesdk/rbenv,cehoffman/nodenv,pyenv/pyenv,sebhahn/pyenv,andrew-christianson/rbenv,princeofdarkness76/rbenv,cyril/rbenv,Angeldude/rbenv,michael-gabenna/rbenv,Angeldude/rbenv,jmatbastos/rbenv,Ferocia/rbenv,takluyver/pyenv,jmatbastos/rbenv,andrew-christianson/rbenv,SamuraiT/rbenv,pumpkineater/rbenv,jrolfs/nodenv,syndbg/goenv,cehoffman/rbenv,sebhahn/pyenv,dnsd/pyenv,josegonzalez/rbenv,RinSer/rbenv,habinmusic/pyenv,syndbg/goenv,chenfengyuan/pyenv,howtolearntocode/rbenv,bradynpoulsen/rbenv,lozybean/pyenv,OiNutter/nodenv,reyesyang/rbenv,sebhahn/pyenv,stivenrgalvis/rbenv,bradynpoulsen/rbenv,beck/rbenv,yyyc514/rbenv,dark5un/nodenv,UCASREN/pyenv,cyril/rbenv,adarshmuthappa/nodenv,rowhit/pyenv,reyesyang/rbenv,dongguangming/pyenv,janusnic/rbenv,jasonkarns/rbenv,SamuraiT/rbenv,chenfengyuan/pyenv,dnsd/pyenv,michael-gabenna/rbenv,rowhit/pyenv,dongguangming/pyenv,syndbg/goenv,intfrr/rbenv,josegonzalez/rbenv,andrew-christianson/rbenv,cehoffman/pyenv