cmd: rename with_keep_cwd to keep_cwd
cmd: rename with_keep_cwd to keep_cwd

Having execute() use a different directory is an important piece
of API information so I added more documentation about it and
renamed the flag to just "keep_cwd" since that's shorter and simpler.

Signed-off-by: David Aguilar <9de348c050f7cd1ca590883733c4e531ce610bf4@gmail.com>

roidelapluie/GitPython,breavo/GitPython,pengshp/GitPython,OpenInkpot-archive/iplinux-python-git,cool-RR/GitPython,h4ck3rm1k3/GitPython,ghtdak/GitPython,hvnsweeting/GitPython,Conjuro/GitPython,expobrain/GitPython,beni55/GitPython,hvnsweeting/GitPython,cdht/GitPython,StyXman/GitPython,gitpython-developers/gitpython,gitpython-developers/GitPython,nvie/GitPython,avinassh/GitPython,manasdk/GitPython,breavo/GitPython,zsjohny/GitPython,roidelapluie/GitPython,cdht/GitPython,avinassh/GitPython,gitpython-developers/gitpython,Conjuro/GitPython,StyXman/GitPython,cool-RR/GitPython,avinassh/GitPython,manasdk/GitPython,expobrain/GitPython,zsjohny/GitPython,nvie/GitPython,bwrsandman/GitPython,zsjohny/GitPython,bwrsandman/GitPython,bwrsandman/GitPython,Conjuro/GitPython,h4ck3rm1k3/GitPython,dpursehouse/GitPython,jeblair/GitPython,beni55/GitPython,pengshp/GitPython,cdht/GitPython,ghtdak/GitPython,StyXman/GitPython,dpursehouse/GitPython,jeblair/GitPython,cool-RR/GitPython,gitpython-developers/GitPython,pengshp/GitPython