testing: diff against the common commit on the master (#3916)
testing: diff against the common commit on the master (#3916)

* testing: diff against the common commit on the master

* Allow git diff to fail

* address code review
GoogleCloudPlatform/python-docs-samples,GoogleCloudPlatform/python-docs-samples,GoogleCloudPlatform/python-docs-samples,GoogleCloudPlatform/python-docs-samples