GitLab

On GitLab.com we are running into issues where batch sizes for the
migration MigrateMergeRequestDiffCommitUsers are too large, resulting in
Sidekiq hosts running out of memory. See issue
https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/14097 for
more details on this.

To work around this, we add a migration that slices the existing jobs
into smaller chunks. This done such that jobs using the old ranges
won't be processed any more. We also change the number of database rows
loaded into memory when migrating data, in an attempt to further reduce
the amount of memory that's necessary.

See https://gitlab.com/gitlab-org/gitlab/-/issues/334394 for more
information.

Changelog: added