diff --git a/src/applications/diffusion/conduit/ConduitAPI_diffusion_branchquery_Method.php b/src/applications/diffusion/conduit/ConduitAPI_diffusion_branchquery_Method.php
index 22f1bb1e4c..4bb7f1b9a9 100644
--- a/src/applications/diffusion/conduit/ConduitAPI_diffusion_branchquery_Method.php
+++ b/src/applications/diffusion/conduit/ConduitAPI_diffusion_branchquery_Method.php
@@ -62,41 +62,16 @@ final class ConduitAPI_diffusion_branchquery_Method
     $drequest = $this->getDiffusionRequest();
     $repository = $drequest->getRepository();
 
-    $refs = id(new DiffusionLowLevelMercurialBranchesQuery())
-      ->setRepository($repository)
-      ->execute();
+    $query = id(new DiffusionLowLevelMercurialBranchesQuery())
+      ->setRepository($repository);
 
-    // If we have a 'contains' query, filter these branches down to just the
-    // ones which contain the commit.
     $contains = $request->getValue('contains');
     if (strlen($contains)) {
-      list($branches_raw) = $repository->execxLocalCommand(
-        'log --template %s --limit 1 --rev %s --',
-        '{branches}',
-        hgsprintf('%s', $contains));
-
-      $branches_raw = trim($branches_raw);
-      if (!strlen($branches_raw)) {
-        $containing_branches = array('default');
-      } else {
-        $containing_branches = explode(' ', $branches_raw);
-      }
-
-      $containing_branches = array_fuse($containing_branches);
-
-      // NOTE: We get this very slightly wrong: a branch may have multiple
-      // heads and we'll keep all of the heads of the branch, even if the
-      // commit is only on some of the heads. This should be rare, is probably
-      // more clear to users as is, and would potentially be expensive to get
-      // right since we'd have to do additional checks.
-
-      foreach ($refs as $key => $ref) {
-        if (empty($containing_branches[$ref->getShortName()])) {
-          unset($refs[$key]);
-        }
-      }
+      $query->withContainsCommit($contains);
     }
 
+    $refs = $query->execute();
+
     return $this->processBranchRefs($request, $refs);
   }
 
diff --git a/src/applications/diffusion/engine/DiffusionCommitHookEngine.php b/src/applications/diffusion/engine/DiffusionCommitHookEngine.php
index 430417fcf7..98bd78bc77 100644
--- a/src/applications/diffusion/engine/DiffusionCommitHookEngine.php
+++ b/src/applications/diffusion/engine/DiffusionCommitHookEngine.php
@@ -688,7 +688,7 @@ final class DiffusionCommitHookEngine extends Phobject {
     foreach (array('old', 'new') as $key) {
       $futures[$key] = $repository->getLocalCommandFuture(
         'heads --template %s',
-        '{node}\1{branches}\2');
+        '{node}\1{branch}\2');
     }
     // Wipe HG_PENDING out of the old environment so we see the pre-commit
     // state of the repository.
@@ -697,7 +697,7 @@ final class DiffusionCommitHookEngine extends Phobject {
     $futures['commits'] = $repository->getLocalCommandFuture(
       'log --rev %s --template %s',
       hgsprintf('%s:%s', $hg_node, 'tip'),
-      '{node}\1{branches}\2');
+      '{node}\1{branch}\2');
 
     // Resolve all of the futures now. We don't need the 'commits' future yet,
     // but it simplifies the logic to just get it out of the way.
@@ -978,15 +978,8 @@ final class DiffusionCommitHookEngine extends Phobject {
     $commits_lines = array_filter($commits_lines);
     $commit_map = array();
     foreach ($commits_lines as $commit_line) {
-      list($node, $branches_raw) = explode("\1", $commit_line);
-
-      if (!strlen($branches_raw)) {
-        $branches = array('default');
-      } else {
-        $branches = explode(' ', $branches_raw);
-      }
-
-      $commit_map[$node] = $branches;
+      list($node, $branch) = explode("\1", $commit_line);
+      $commit_map[$node] = array($branch);
     }
 
     return $commit_map;
@@ -1152,6 +1145,9 @@ final class DiffusionCommitHookEngine extends Phobject {
       case PhabricatorRepositoryType::REPOSITORY_TYPE_GIT:
         return idx($this->gitCommits, $identifier, array());
       case PhabricatorRepositoryType::REPOSITORY_TYPE_MERCURIAL:
+        // NOTE: This will be "the branch the commit was made to", not
+        // "a list of all branch heads which descend from the commit".
+        // This is consistent with Mercurial, but possibly confusing.
         return idx($this->mercurialCommits, $identifier, array());
       case PhabricatorRepositoryType::REPOSITORY_TYPE_SVN:
         // Subversion doesn't have branches.
diff --git a/src/applications/diffusion/query/lowlevel/DiffusionLowLevelMercurialBranchesQuery.php b/src/applications/diffusion/query/lowlevel/DiffusionLowLevelMercurialBranchesQuery.php
index f779794b51..015f0b1c8e 100644
--- a/src/applications/diffusion/query/lowlevel/DiffusionLowLevelMercurialBranchesQuery.php
+++ b/src/applications/diffusion/query/lowlevel/DiffusionLowLevelMercurialBranchesQuery.php
@@ -6,21 +6,36 @@
 final class DiffusionLowLevelMercurialBranchesQuery
   extends DiffusionLowLevelQuery {
 
+  private $contains;
+
+  public function withContainsCommit($commit) {
+    $this->contains = $commit;
+    return $this;
+  }
+
   protected function executeQuery() {
     $repository = $this->getRepository();
 
-    // NOTE: `--debug` gives us 40-character hashes.
+    if ($this->contains !== null) {
+      $spec = hgsprintf('(descendants(%s) and head())', $this->contains);
+    } else {
+      $spec = hgsprintf('head()');
+    }
+
     list($stdout) = $repository->execxLocalCommand(
-      '--debug branches');
-    $stdout = PhabricatorRepository::filterMercurialDebugOutput($stdout);
+      'log --template %s --rev %s',
+      '{node}\1{branch}\2',
+      $spec);
 
     $branches = array();
 
-    $lines = ArcanistMercurialParser::parseMercurialBranches($stdout);
-    foreach ($lines as $name => $spec) {
+    $lines = explode("\2", $stdout);
+    $lines = array_filter($lines);
+    foreach ($lines as $line) {
+      list($node, $branch) = explode("\1", $line);
       $branches[] = id(new DiffusionRepositoryRef())
-        ->setShortName($name)
-        ->setCommitIdentifier($spec['rev']);
+        ->setShortName($branch)
+        ->setCommitIdentifier($node);
     }
 
     return $branches;