Ignore PRs with large numbers of commits

[gsnedders]

If there's a push with >100 commits, it's almost always a rebase gone wrong, and it doesn't make sense to add labels and reviewers on that basis.

web-platform-tests/wpt#53459 is the most recent example I've seen, but this has been a problem for a long time.

[mertcanaltin]

Thanks for opening this! I can confirm this is exactly what happened with #53459 - I was doing a rebase and it went sideways, ending up with 100+ commits and tagging everyone unnecessarily.

Definitely agree that the bot should skip auto-labeling and reviewer assignment for PRs with an unusually high commit count. It would save everyone from the notification spam and make it easier to spot when something actually needs attention vs. when it's just a rebase gone wrong.