R1:e2cf1e42887a

Skip copied code detection for changes that are too large for it to be useful

Summary:
Ref T13210. See PHI944. When parsing certain large diffs (the case in PHI944 is an 2.5-million line JSON diff), we spend ~66% of runtime and ~80% of memory doing copy detection (the little yellow bar which shows up to give you a hint that code was moved around within a diff).

This is pretty much pointless and copy hints are almost certainly never useful on large changes. Instead, just bail if the change is larger than…