sync: use subprocess.run to verify tags

The code is a bit simpler & easier to reason about.

Change-Id: I149729c7d01434b08b58cc9715dcf0f0d11201c2
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/297022


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