The background migration FixMergeRequestDiffCommitUsers, originally
added in https://gitlab.com/gitlab-org/gitlab/-/merge_requests/73307,
didn't use keyset pagination for iterating over diff commits. In some
cases this would lead to a SQL timeout, as a single query would try to
fetch too many diff commit rows at once. This commit changes the code to
use keyset pagination, allowing us to fetch the necessary data without
any SQL timeouts.

Changelog: performance