ProcessCommitWorker, via /api/v4/internal/post_receive, can in certain circumstances result in 100K+ Gitaly calls for a single repository

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/65381 - except for ProcessCommitWorker instead of ProjectCacheWorker (it might be worth investigating this as a single issue)

In certain circumstances, a single /api/v4/internal/post_receive can lead to 100k n + 1 FindCommit Gitaly calls onto the underlying repository from the Sidekiq ProcessCommitWorker job.

This is also leading to the Gitaly Abuse detector alerting on possible abuse (it is not)

Example correlation_id for which this occurs: https://log.gitlab.net/goto/ffe40d08969d41fc9ed8152fede60290

Example abuse report: https://gitlab.slack.com/archives/CCR9GMMR7/p1564643077043900