Support an "Ancestors Of: ..." constraint in commit queries

Summary:
Ref T13137. See PHI609. An install would like to filter audit requests on a particular branch, e.g. "master".

This is difficult in the general case because we can not apply this constraint efficiently under every conceivable data shape, but we can do a reasonable job in most practical cases.

See T13137#238822 for more detailed discussion on the approach here.

This is a bit rough, but should do the job for now.

Test Plan:
- Filtered commits by various branches, e.g. "master"; "lfs". Saw correct-seeming results.
- Stubbed out the "just list everything" path to hit the `diffusion.internal.ancestors` path, saw the same correct-seeming results.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13137

Differential Revision: https://secure.phabricator.com/D19431

src/applications/audit/query/PhabricatorCommitSearchEngine.php

@@ -53,6 +53,10 @@ final class PhabricatorCommitSearchEngine
       $query->withUnreachable($map['unreachable']);
     }
 
+    if ($map['ancestorsOf']) {
+      $query->withAncestorsOf($map['ancestorsOf']);
+    }
+
     return $query;
   }
 
@@ -103,6 +107,13 @@ final class PhabricatorCommitSearchEngine
           pht(
             'Find or exclude unreachable commits which are not ancestors of '.
             'any branch, tag, or ref.')),
+      id(new PhabricatorSearchStringListField())
+        ->setLabel(pht('Ancestors Of'))
+        ->setKey('ancestorsOf')
+        ->setDescription(
+          pht(
+            'Find commits which are ancestors of a particular ref, '.
+            'like "master".')),
     );
   }