git-combine: Fix too many commits bug

Created by: chrismwendt

There are over 10M commits on the megarepo right now 🙀

This is because I introduced some logic in https://github.com/sourcegraph/sourcegraph/pull/31832 that interacted poorly with the 1K commit depth limit.

cc @tsenart in case you'd like to review this.

Test plan

Ran locally.