git: Add support for GIT_SSH_COMMAND environment variable
git: Add support for GIT_SSH_COMMAND environment variable

Git 2.3 supports GIT_SSH_COMMAND as a way to specify command line
parameters to pass to git. It is usually used as a way to specify a
particular identity.

`GIT_SSH_COMMAND='ssh -i /path/to/identity' npm install`

cchamberlain/npm-msys2,rsp/npm,DIREKTSPEED-LTD/npm,misterbyrne/npm,lxe/npm,thomblake/npm,kemitchell/npm,haggholm/npm,cchamberlain/npm,DaveEmmerson/npm,TimeToogo/npm,midniteio/npm,segrey/npm,DaveEmmerson/npm,DIREKTSPEED-LTD/npm,yibn2008/npm,princeofdarkness76/npm,haggholm/npm,xalopp/npm,ekmartin/npm,kimshinelove/naver-npm,DIREKTSPEED-LTD/npm,evocateur/npm,kemitchell/npm,evocateur/npm,misterbyrne/npm,yodeyer/npm,yibn2008/npm,haggholm/npm,evanlucas/npm,midniteio/npm,TimeToogo/npm,Volune/npm,yodeyer/npm,princeofdarkness76/npm,segrey/npm,evanlucas/npm,thomblake/npm,midniteio/npm,cchamberlain/npm,xalopp/npm,cchamberlain/npm-msys2,yodeyer/npm,segrey/npm,cchamberlain/npm-msys2,chadnickbok/npm,rsp/npm,lxe/npm,chadnickbok/npm,lxe/npm,ekmartin/npm,Volune/npm,cchamberlain/npm,rsp/npm,DaveEmmerson/npm,thomblake/npm,kimshinelove/naver-npm,TimeToogo/npm,princeofdarkness76/npm,kemitchell/npm,ekmartin/npm,yibn2008/npm,evanlucas/npm,xalopp/npm,Volune/npm,chadnickbok/npm,misterbyrne/npm,evocateur/npm,kimshinelove/naver-npm