add global limit for commit/diff search with paged repos

Created by: camdencheek

Previously, we would resolve all the repos at once, then check limits on them and error if there were to many repos being searched. Now, we run commit searches 500 repos at a time, which will always be below our limits.

This is a problem because on sourcegrpah.com, some monitors will search over every repo looking for something that doesn't exist. This is very expensive and will time out every time.

Ideally, we will clean up the garbage code monitors on sourcegraph.com and remove limits on commit/diff search (other than timeouts), but for now, this bandaid should help reduce load.