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>