Improve ProcessCommitWorker for large push payloads
This adjusts ProcessCommitWorker so that it processes at most the 100 most recent commits of a push payload.
Limiting commits ensures one can't overload the system by pushing a payload containing e.g. 30 000 commits (or more). Since most "genuine" pushes will only contain a dozen or so commits a limit of 100 should suffice.
Fixes #25827 (closed)