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

## What are the relevant issue numbers?

Closes #22229

See merge request !6435