repo: add --version support to the launcher

We can get version info when in a checkout, but it'd be helpful
to show that info at all times.

Change-Id: Ieeb44a503c9d7d8c487db4810bdcf3d5f6656c82
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254712
Reviewed-by: Michael Mortensen <mmortensen@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Mike Frysinger <vapier@google.com>