Set GIT_WORK_TREE by default in _git_env() if not a bare repository
Set GIT_WORK_TREE by default in _git_env() if not a bare repository

This makes the checkout() function work as expected without changing
the current working directory while still allowing config_get() to
call _git_env(False) in order to delete GIT_WORK_TREE from the
environment to avoid an infinite recursion (and because "git config"
works without a working tree).

ustuehler/git-cvs,ustuehler/git-cvs