Fix a Mercurial issue where split heads would be detected incorrectly

Summary: Ref T5197. When searching for split branch heads, we incorrectly consider descendant heads of other branches. This can cause us to detect a split tip when one does not exist (the old tip is the branch tip, but other descendant heads exist). Instead, consider only heads on the same branch.

Test Plan:
Repro is something like this:

  - `hg update default`
  - `hg branch branch1; hg commit ...`
  - `hg push`
  - `hg update default; hg commit ...`
  - `hg push` - Previously, we would find the head of `branch1` and incorrectly account for it as a head of `default`.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5197

Differential Revision: https://secure.phabricator.com/D9308

src/applications/diffusion/engine/DiffusionCommitHookEngine.php

@@ -760,10 +760,15 @@ final class DiffusionCommitHookEngine extends Phobject {
         // repository that's already full of garbage (strongly discouraged but
         // not as inherently dangerous). These cases should be very uncommon.
 
+        // NOTE: We're only looking for heads on the same branch. The old
+        // tip of the branch may be the branchpoint for other branches, but that
+        // is OK.
+
         $dfutures = array();
         foreach ($old_heads as $old_head) {
           $dfutures[$old_head] = $repository->getLocalCommandFuture(
-            'log --rev %s --template %s',
+            'log --branch %s --rev %s --template %s',
+            $ref,
             hgsprintf('(descendants(%s) and head())', $old_head),
             '{node}\1');
         }