project: fix rebase check with worktrees

Add a helper to our git wrapper to find the .git subdir,
and then use that to detect internal rebase state.

Change-Id: I3b3b6ed4c1f45cc8c3c98dc19c7ca3aabdc46905
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/256532
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>