Fix several error handling issues with Subversion commits in Diffusion

Summary:
Ref T9513. I checked this briefly but didn't do a very thorough job of it.

  - Don't try to query merges for Subversion, since it doesn't support them.
  - Fix up "existsquery" to work properly (and efficiently) for both hosted and imported repositories.
  - Fix up "parentsquery" to have similar behavior on invalid commits to other VCSes (throw an exception).

Test Plan:
  - No more merges warning on SVN.
  - Hosted SVN gets the right exists result now.
  - Visiting "r23980283789287" now 404's instead of "not parsed yet".

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9513

Differential Revision: https://secure.phabricator.com/D14239

src/applications/diffusion/controller/DiffusionCommitController.php

@@ -1136,14 +1136,17 @@ final class DiffusionCommitController extends DiffusionController {
     $merge_limit = $this->getMergeDisplayLimit();
 
     try {
-      $merges = $this->callConduitWithDiffusionRequest(
-        'diffusion.mergedcommitsquery',
-        array(
-          'commit' => $commit,
-          'limit' => $merge_limit + 1,
-        ));
-
-      $this->commitMerges = DiffusionPathChange::newFromConduit($merges);
+      if ($repository->isSVN()) {
+        $this->commitMerges = array();
+      } else {
+        $merges = $this->callConduitWithDiffusionRequest(
+          'diffusion.mergedcommitsquery',
+          array(
+            'commit' => $commit,
+            'limit' => $merge_limit + 1,
+          ));
+        $this->commitMerges = DiffusionPathChange::newFromConduit($merges);
+      }
     } catch (Exception $ex) {
       $this->commitMerges = false;
       $exceptions[] = $ex;