Add update_diff_fixtures script
Add update_diff_fixtures script

Adds a convenience script for generating and submitting a patch for
updating the diff fixtures.

Change-Id: Ibe4de245de6bbfba4a1683c75d7b50e82221c928

wikimedia/mediawiki-services-mobileapps,wikimedia/mediawiki-services-mobileapps,wikimedia/mediawiki-services-mobileapps,wikimedia/mediawiki-services-mobileapps,wikimedia/mediawiki-services-mobileapps,wikimedia/mediawiki-services-mobileapps