Add "diffusion.commit.edit", a v3 edit API endpoint for commits

Summary: Ref T10978. This currently does almost nothing, but gets it in place so I can add stuff to it.

Test Plan: Made a comment on a commit using the API.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10978

Differential Revision: https://secure.phabricator.com/D17178

src/applications/diffusion/conduit/DiffusionCommitEditConduitAPIMethod.php

@@ -0,0 +1,20 @@
+<?php
+
+final class DiffusionCommitEditConduitAPIMethod
+  extends PhabricatorEditEngineAPIMethod {
+
+  public function getAPIMethodName() {
+    return 'diffusion.commit.edit';
+  }
+
+  public function newEditEngine() {
+    return new DiffusionCommitEditEngine();
+  }
+
+  public function getMethodSummary() {
+    return pht(
+      'Apply transactions to edit an existing commit. This method can not '.
+      'create new commits.');
+  }
+
+}

src/applications/diffusion/editor/DiffusionCommitEditEngine.php

@@ -26,7 +26,16 @@ final class DiffusionCommitEditEngine
   }
 
   protected function newEditableObject() {
-    throw new PhutilMethodNotImplementedException();
+    // NOTE: We must return a valid object here so that things like Conduit
+    // documentation generation work. You can't actually create commits via
+    // EditEngine. This is enforced with a "No One" creation policy.
+
+    $repository = new PhabricatorRepository();
+    $data = new PhabricatorRepositoryCommitData();
+
+    return id(new PhabricatorRepositoryCommit())
+      ->attachRepository($repository)
+      ->attachCommitData($data);
   }
 
   protected function newObjectQuery() {