filter MR log with too many rebases

Problem to solve

Some MRs are subject to too many rebases, causing a lot of noise in the log.

See https://salsa.debian.org/debian/devscripts/merge_requests/70 for such a log. It takes a long time to load, and then requires a lot of scrolling through, with quite a chance of losing important messages in between.

Proposal

coalescing multiple "XXX added X commits" messages when they are near each other should be a simple matter and at the same time quite effective already.