Use base SHA for patches and diffs

What does this MR do?

Switch from using 'start SHA' to 'base SHA' for patches and diffs

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Makes the downloaded patches and diffs on the merge request page match the frontend-rendered "changes" in these scenarios:

• Unpatched gitlab-workhorse, downloading patchsets of open MRs (https://gitlab.com/gitlab-org/gitlab-workhorse/merge_requests/68)
• Unpatched gitlab-workhorse, downloading diffs of open and merged MRs
• Patched gitlab-workhorse, downloading patchsets of merged merge requests

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• [ ] Documentation created/updated
• [ ] API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #22229 (closed)