Allow commits to be queried by repository using the tagged(...) typehaead function

Summary:
Fixes T12322. Allows you to search for commits using the `tagged(...)` repository function, so you can find "any commmit in any repository tagged with android" or similar.

I moved the function from Differential (which was the application using it) to Diffusion (which is more accurately the application which provides it).

I fixed a bug where searching for `tagged(xyz)` would have no effect (constraint was ignored) if there were no repositories tagged with "xyz". The fix isn't perfectly clean, but should work properly for the moment.

Test Plan:
  - Searched with `tagged(...)` in Diffusion and Differential.
  - Searched by repository.
  - Searched with `tagged(...)` for a project with no tagged repositories.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12322

Differential Revision: https://secure.phabricator.com/D17426

src/applications/diffusion/typeahead/DiffusionRepositoryFunctionDatasource.php

@@ -0,0 +1,25 @@
+<?php
+
+final class DiffusionRepositoryFunctionDatasource
+  extends PhabricatorTypeaheadCompositeDatasource {
+
+  public function getBrowseTitle() {
+    return pht('Browse Repositories');
+  }
+
+  public function getPlaceholderText() {
+    return pht('Type a repository name or function...');
+  }
+
+  public function getDatasourceApplicationClass() {
+    return 'PhabricatorDifferentialApplication';
+  }
+
+  public function getComponentDatasources() {
+    return array(
+      new DiffusionTaggedRepositoriesFunctionDatasource(),
+      new DiffusionRepositoryDatasource(),
+    );
+  }
+
+}

src/applications/diffusion/typeahead/DiffusionTaggedRepositoriesFunctionDatasource.php

@@ -67,11 +67,19 @@ final class DiffusionTaggedRepositoriesFunctionDatasource
       ->execute();
 
     $results = array();
-
     foreach ($repositories as $repository) {
       $results[] = $repository->getPHID();
     }
 
+    if (!$results) {
+      // TODO: This is a little hacky, but if you query for "tagged(x)" and
+      // there are no such repositories, we want to match nothing. If we
+      // just return `array()`, that gets evaluated as "no constraint" and
+      // we match everything. This works correctly for now, but should be
+      // replaced with some more elegant/general approach eventually.
+      $results[] = PhabricatorPHIDConstants::PHID_VOID;
+    }
+
     return $results;
   }