Fix issue with gerrit_util.py and add tests.

Change-Id: Ie523ea59ddb93cf5c7fa35f3761310ce96747720
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2081092Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>