version: include tag commit date for easy reference

This is more for users trying to get a sense of how old/new their
current version of repo is when debugging issues.

Change-Id: Ifb413c679bb8c8dbf4f9334137adf086bb000a68
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/261192
Reviewed-by: Jonathan Nieder <jrn@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

subcmds/version.py

@@ -43,6 +43,7 @@ class Version(Command, MirrorSafeCommand):
     rp_ver = rp.bare_git.describe(HEAD)
     print('repo version %s' % rp_ver)
     print('       (from %s)' % rem.url)
+    print('       (%s)' % rp.bare_git.log('-1', '--format=%cD', HEAD))
 
     if self.wrapper_path is not None:
       print('repo launcher version %s' % self.wrapper_version)