project: unify HEAD path management

Add a helper function to unify the duplication of finding the full
path to the symbolic HEAD ref.  This makes it easy to handle git
worktrees where .git is a file rather than a dir/symlink.

Bug: https://crbug.com/gerrit/11486
Change-Id: I9f794f1295ad0d98c7c13622f01ded51e4ba7846
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254074


Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>