git cl tests: refactor gerrit base calls for readability.
git cl tests: refactor gerrit base calls for readability.

R=bbed39beedae4cdb499af742d2fcd6c63934d93b@chromium.org
BUG=689652

Change-Id: I3074eeef2b939d64bcd2e537bbc7ee288241bd79
Reviewed-on: https://chromium-review.googlesource.com/443108
Commit-Queue: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org>
Reviewed-by: Aaron Gable <bbed39beedae4cdb499af742d2fcd6c63934d93b@chromium.org>

CoherentLabs/depot_tools,CoherentLabs/depot_tools