UpdateMergeRequestsWorker calling CommitIsAncestor 2000+ times

From https://log.gitlab.net/goto/c815fcdf7824c08ddef6ace876bdf909, it looks like when a user merges to master on www-gitlab-com, this incurs over 2,000 CommitIsAncestor Gitaly calls. The parameters were:

["7764","2920426","d4ebe7327b7141a6749103aaec4c0e20eadd40e0","91f1b6478ef833e99bfd5a3e5d8beab7a9030368","refs/heads/master"]