R1:c6fc05ee0941

Pull Git filesize logic into a separate LowLevel query and use more Iterators

Summary:
Depends on D19829. Ref T13216. See PHI908. The current implementation is kind of a lot to live in `CommitHookEngine` and will likely fail if `git diff-tree` produces more than 2GB of output.

Pull it out and make it slightly more robust against enormous commits. It's probably limited by this, now:

```
implode("\n", $every_path)
```

We could replace that with some `PhutilReverseRopeSource` primitive or something but…