subrepo: use subprocess's cwd instead of git's --work-tree Some older git commands (e.g. git merge) do not properly recognize the --work-tree argument, so abstract that away from git.

test-subrepo-git.t: make gitroot pushable earlier Older git versions get confused when the git repo becomes bare.

test-subrepo-git.t: silence git output Older git versions have different ideas of what output is considered quiet, so the tests can be more version-blind.