Test-only job shouldn't require constant rebases to detect which files were changed.

Problem/Motivation

Currently, the test-only job relies on a rebased MR to detect the changes and then do the rest of the logic. If the MR is not rebased, it will pick up other files not changed at all in the current MR.

Steps to reproduce

Create an MR against 11.x, wait until the next commit in 11.x, then run the "Test-only" job, you'll see that the list of files changed shown in the job contains more files than the ones that were changed in the MR.

Proposed resolution

Use a different "git diff" logic to detect the exact changes of the MR.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet