Reindexing on large btrees to clean up bloat from migration

Following up with https://gitlab.com/gitlab-org/database-team/team-tasks/-/issues/201#note_716362912:

Schedule reindex operations for index_merge_request_diff_commits_on_sha and merge_request_diff_commits_pkey.

Goals:

1. Remove bloat, free up some 400 GB
2. See if there is any impact from very long reindexing operations (if not: follow-up with increasing the reindexing limit in https://gitlab.com/gitlab-org/gitlab/blob/master/lib/gitlab/database/reindexing/index_selection.rb#L13)