Regression tests / end-to-end tests that ensure diff, commit, and repo search work do not exist or are insufficient

Created by: slimsag

I introduced a major regression by accident in 3.10.2 which entirely broke diff/commit/repo search: #7090 (closed)

The release process for patches, as well as our CI system (end-to-end tests) did not indicate that this regression occurred and as such the release went ahead. We should be catching such a major regression.

cc @sourcegraph/distribution @beyang