Remove commit from DiffusionRequest

Summary: Ref T2683. This field is //almost// entirely redundant with `symbolicCommit`. Improve how some of the diff query stuff works a bit, then remove it.

Test Plan: Browsed around in all interfaces, looked at a bunch of diffs, etc.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2683

Differential Revision: https://secure.phabricator.com/D9099

src/applications/diffusion/controller/DiffusionDiffController.php

@@ -51,7 +51,7 @@ final class DiffusionDiffController extends DiffusionController {
       array(
         'commit' => $drequest->getCommit(),
         'path' => $drequest->getPath()));
-    $drequest->setCommit($data['effectiveCommit']);
+    $drequest->updateSymbolicCommit($data['effectiveCommit']);
     $raw_changes = ArcanistDiffChange::newFromConduit($data['changes']);
     $diff = DifferentialDiff::newFromRawChanges($raw_changes);
     $changesets = $diff->getChangesets();