R1:de999af61422

Improve some behaviors around memory pressure when pushing many and/or large changes

Summary:
Ref T13142. When commits are pushed, we try to handle them on one of two pathways:

- Normal changes: we load these into memory and potentially apply Herald content rules to them.
- "Enormous" changes: we don't load these into memory and skip content rules for them.

The goal is to degrade gracefully when users push huge changes: they should work, just not support all the features.

However, some changes can…