MR diffs batch loading rollout plan

The first release attempt for &1816 (closed) is 12.6. It puts together quite a few MRs and changes behind a feature flag that need some coordination for rollout in GitLab.com. So here's the roadmap:

Required changes

• Merge MRs for #31290 (closed) behind diffs_batch_load feature flag
• Merge the new Redis cache using hset !19917 (merged)
• !21184 (merged)
• !21210 (merged) @oswaldo
• !21124 (merged) (bug fix)
• !20980 (merged) blocker (we need to load 20 files per request)
• #39095 (closed) blocker (bug)
• #39104 (closed) blocker (for enabling it with the new cache) @oswaldo & @kerrizor
• #39133 (closed) blocker (for enabling it with the new cache) @oswaldo
• !21446 (merged) blocker (diffs call can loop forever)
• !22982 (merged)
• #197402 (closed) blocker

Good to have

• !20939 (merged) @oswaldo
• #43654 (closed) @oswaldo
• !20920 (merged) @iamphill - ?
• !19930 (merged) - Really nice to have, cuts quite a few seconds from the requests (@thomasrandolph) - ?

To get done before the 17th?

• #38008 (closed)
• #38106 (closed)
• !23105 (merged)

Testing and what to be aware of

• Enable just the new cache (hset_redis_diff_caching). No performance decrease or Redis consumption bump should be perceived
• Ideally we should enable the feature (diffs_batch_load) for personal projects, (if 👍) then small GitLab.com projects, (if 👍 ) gitlab-org group or gitlab project. Let's leave it enabled that way for some time and see if anything comes up.