R1:53b25db91856

Prevent enormous changes from being pushed to repositoires by default

Summary:
Fixes T13031. "Enormous" changes are basically changes which are too large to hold in memory, although the actual definition we use today is "more than 1GB of change text or `git diff` runs for more than 15 minutes".

If an install configures a Herald content rule like "when content matches /XYZ/, do something" and then a user pushes a 30 GB source file, we can't put it into memory to `preg_match()` it. Currently, the way to…