Remove serialised diff and commit columns

Closes #39533

See merge request gitlab-org/gitlab-ce!15582

Rescheduling will make sure the fork networks with a deleted source
project are created.

For getting the SHAs from an MR to find pipelines, we get the last 100 MR diffs
for the MR, and find commits from those. This was un-indexed before, because the
index was not a composite index on merge_request_diff_id, id. Changing that
means that this scope can exclusively use indexes.

The st_commits and st_diffs columns on merge_request_diffs historically held the
YAML-serialised data for a merge request diff, in a variety of formats.

Since 9.5, these have been migrated in the background to two new tables:
merge_request_diff_commits and merge_request_diff_files. That has the advantage
that we can actually query the data (for instance, to find out how many commits
we've stored), and that it can't be in a variety of formats, but must match the
new schema.

This is the final step of that journey, where we drop those columns and remove
all references to them. This is a breaking change to the importer, because we
can no longer import diffs created in the old format, and we cannot guarantee
the export will be in the new format unless it was generated after this commit.

Adds Rubocop rule for line break after guard clause

Closes #18040

See merge request gitlab-org/gitlab-ce!15188

Restrict update column in batches for large tables

See merge request gitlab-org/gitlab-ce!15458

add_column_with_default is implemented in terms of update_column_in_batches, but
update_column_in_batches can be used independently. Neither of these should be
used on the specified large tables, because they will cause issues on large
instances like GitLab.com.

This also ignores the cop for all existing migrations, renaming
AddColumnWithDefaultToLargeTable where appropriate.

Delete empty fork networks

See merge request gitlab-org/gitlab-ce!15373