Clear caches before updating MR diffs

See merge request gitlab-org/gitlab-ce!15916

The hook ordering influenced the diffs being generated as these used
values from before the update due to the memoization still being in
place. This commit reorders them and tests against this behaviour.

Update issuable status icons

Closes #33926

See merge request gitlab-org/gitlab-ce!15898

Prevent worker that updates merge requests head pipeline from failing jobs

Closes #41021

See merge request gitlab-org/gitlab-ce!15870

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/33926

Changed file icons already addressed in
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15367

The Gitaly CommitService is being hammered by n + 1 calls, mostly when
finding commits. This leads to this gRPC being turned of on production:
https://gitlab.com/gitlab-org/gitaly/issues/514#note_48991378

Hunting down where it came from, most of them were due to
MergeRequest#show. To prove this, I set a script to request the
MergeRequest#show page 50 times. The GDK was being scraped by
Prometheus, where we have metrics on controller#action and their Gitaly
calls performed. On both occations I've restarted the full GDK so all
caches had to be rebuild.

Current master, 806a68a81f1baee, needed 435 requests
After this commit, 154 requests

the `ci_pipelines.sha` column is not the same type than the
`merge_request_diff_commits.sha` column (varchar, bytea)