Return Diffusion diffs through Files, not directly over Conduit

Summary:
Fixes T10423. Ref T11524. This changes `diffusion.rawdiffquery` to return a file PHID instead of a blob of data.

This is better in general, but particularly better for huge diffs (as in T10423) and diffs with non-utf8 data (as in T10423).

Test Plan:
  - Used `bin/differential extract` to extract a latin1 diff, got a clean diff.
  - Used `bin/repository reparse --herald` to rerun herald on a latin1 diff, got a clean result.
  - Pushed latin1 diffs to test commit hooks.
  - Triggered the the too large / too slow logic.
  - Viewed latin1 diffs in Diffusion.
  - Used "blame past this change" in Diffusion to hit the `before` logic.

Reviewers: chad

Reviewed By: chad

Subscribers: eadler

Maniphest Tasks: T10423, T11524

Differential Revision: https://secure.phabricator.com/D16460

src/applications/diffusion/engine/DiffusionCommitHookEngine.php

@@ -1095,7 +1095,7 @@ final class DiffusionCommitHookEngine extends Phobject {
           ->setTimeout($time_limit)
           ->setByteLimit($byte_limit)
           ->setLinesOfContext(0)
-          ->loadRawDiff();
+          ->executeInline();
         break;
       case PhabricatorRepositoryType::REPOSITORY_TYPE_SVN:
         // TODO: This diff has 3 lines of context, which produces slightly