Expose "identifiers" as a query constraint for Commit search

Summary: Ref T13216. See PHI984. The CommitSearchEngine (and, by extension, `diffusion.commit.search`) currently do not support identifier search, but this is a reasonable capability to provide.

Test Plan:
Testing that a commit exists on `master`:

{F6020742}

Same commit is not on `stable`:

{F6020743}

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13216

Differential Revision: https://secure.phabricator.com/D19825

src/applications/audit/query/PhabricatorCommitSearchEngine.php

@@ -58,6 +58,10 @@ final class PhabricatorCommitSearchEngine
       $query->withAncestorsOf($map['ancestorsOf']);
     }
 
+    if ($map['identifiers']) {
+      $query->withIdentifiers($map['identifiers']);
+    }
+
     return $query;
   }
 
@@ -130,6 +134,15 @@ final class PhabricatorCommitSearchEngine
           pht(
             'Find commits which are ancestors of a particular ref, '.
             'like "master".')),
+      id(new PhabricatorSearchStringListField())
+        ->setLabel(pht('Identifiers'))
+        ->setKey('identifiers')
+        ->setDescription(
+          pht(
+            'Find commits with particular identifiers (usually, hashes). '.
+            'Supports full or partial identifiers (like "abcd12340987..." or '.
+            '"abcd1234") and qualified or unqualified identifiers (like '.
+            '"rXabcd1234" or "abcd1234").')),
     );
   }