project: allow CurrentBranch to return None on errors

If the repo client checkout is in an incomplete sync state, the work
git repo might be in a bad way.  Turn errors parsing HEAD into None
since callers of CurrentBranch already need to account for it.

Change-Id: Ia7682e29ef4182006b1fb5f5e57800f8ab67a9f4
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/239239
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>