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 since we don't have one of those and it seems unlikely that we'll hit this case in practice, I left it here for now with just the easy stuff converted to be stream-oriented.

Test Plan:
Used this script to test the query against various commits, got good results:

```
<?php

require_once 'scripts/init/init-script.php';

$viewer = PhabricatorUser::getOmnipotentUser();

$repository = id(new PhabricatorRepositoryQuery())
->setViewer($viewer)
->withCallsigns(array('P'))
->executeOne();

var_dump(
id(new DiffusionLowLevelFilesizeQuery())
->setRepository($repository)
->withIdentifier($argv[1])
->execute());
```

Used this to find large commits in history and pull filesizes (worked great, although our largest commit only touches a couple thousand paths):

```
for hash in `git log --format=%H`; do echo -n $hash; echo -n ' '; git diff-tree -r --no-commit-id $hash | wc -l | awk '{print $1}'; done | awk '{print $2 " " $1}' | sort -n
```

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13216

Differential Revision: https://secure.phabricator.com/D19830