Allow users to create buildables from diffs

Summary:
Now that diffs have PHIDs we can create buildables for them.

This also adds `buildable.diff` in the variables list so the diff ID is available, and it also fixes the Cancel button on "Edit Plan" page so it redirects to the right place.

Test Plan: Created a buildable from a diff, ran a build plan against it that had `echo ${buildable.diff}` and got the right ID.  Also tested the "Edit Plan" cancel redirect.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D7546

src/applications/harbormaster/controller/HarbormasterBuildableEditController.php

@@ -42,8 +42,11 @@ final class HarbormasterBuildableEditController
           ->executeOne();
 
         if ($object instanceof DifferentialRevision) {
-          throw new Exception(
-            "TODO: We need to assign PHIDs to diffs before this will work.");
+          $revision = $object;
+          $object = $object->loadActiveDiff();
+          $buildable
+            ->setBuildablePHID($object->getPHID())
+            ->setContainerPHID($revision->getPHID());
         } else if ($object instanceof PhabricatorRepositoryCommit) {
           $buildable
             ->setBuildablePHID($object->getPHID())

src/applications/harbormaster/controller/HarbormasterPlanEditController.php

@@ -68,7 +68,7 @@ final class HarbormasterPlanEditController
       $id = $plan->getID();
 
       $title = pht('Edit Build Plan');
-      $cancel_uri = "/B{$id}";
+      $cancel_uri = $this->getApplicationURI('plan/'.$plan->getID().'/');
       $save_button = pht('Save Build Plan');
     }
 

src/applications/harbormaster/step/VariableBuildStepImplementation.php

@@ -4,6 +4,7 @@ abstract class VariableBuildStepImplementation extends BuildStepImplementation {
 
   public function retrieveVariablesFromBuild(HarbormasterBuild $build) {
     $results = array(
+      'buildable.diff' => null,
       'buildable.revision' => null,
       'buildable.commit' => null,
       'repository.callsign' => null,
@@ -17,6 +18,7 @@ abstract class VariableBuildStepImplementation extends BuildStepImplementation {
 
     $repo = null;
     if ($object instanceof DifferentialDiff) {
+      $results['buildable.diff'] = $object->getID();
       $revision = $object->getRevision();
       $results['buildable.revision'] = $revision->getID();
       $repo = $revision->getRepository();
@@ -72,6 +74,8 @@ abstract class VariableBuildStepImplementation extends BuildStepImplementation {
 
   public function getAvailableVariables() {
     return array(
+      'buildable.diff' =>
+        pht('The differential diff ID, if applicable.'),
       'buildable.revision' =>
         pht('The differential revision ID, if applicable.'),
       'buildable.commit' => pht('The commit identifier, if applicable.'),