start: add a --HEAD alias

This makes it easy to use --H as a shortcut, and kind of matches the
use of storing HEAD as the revision.

Change-Id: I590bf488518f313e7a593853140316df98262d7e
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/301163


Reviewed-by: Michael Mortensen <mmortensen@google.com>
Tested-by: Mike Frysinger <vapier@google.com>