Detect if a commit *really* doesn't exist and 4oh4 from Diffusion commit view

Summary: rather than showing an erroneous "we still parsing" message.

Test Plan: for each version control system, typed in a garbage URL and got a 4oh4. (note this actually fails for SVN -- see comment about how my code fails atm -- and DiffusionGitRequest seems to pick off this error in advance and returns an AphrontUsageException.)

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1624

Differential Revision: https://secure.phabricator.com/D3201

src/applications/diffusion/controller/DiffusionCommitController.php

@@ -50,13 +50,16 @@ final class DiffusionCommitController extends DiffusionController {
     $commit = $drequest->loadCommit();
 
     if (!$commit) {
-      // TODO -- T1624 -- detect if this has actually not been parsed yet
-      // and show this UI if so, else 404
+      $query = DiffusionExistsQuery::newFromDiffusionRequest($drequest);
+      $exists = $query->loadExistentialData();
+      if (!$exists) {
+        return new Aphront404Response();
+      }
       return $this->buildStandardPageResponse(
         id(new AphrontErrorView())
         ->setTitle('Error displaying commit.')
-        ->appendChild('Failed to load the commit. The commit has not been '.
-                      'parsed yet.'),
+        ->appendChild('Failed to load the commit because the commit has not '.
+                      'been parsed yet.'),
           array('title' => 'Commit Still Parsing')
         );
     }