diff --git a/src/applications/diffusion/conduit/DiffusionBranchQueryConduitAPIMethod.php b/src/applications/diffusion/conduit/DiffusionBranchQueryConduitAPIMethod.php
--- a/src/applications/diffusion/conduit/DiffusionBranchQueryConduitAPIMethod.php
+++ b/src/applications/diffusion/conduit/DiffusionBranchQueryConduitAPIMethod.php
@@ -30,18 +30,31 @@
 
     $contains = $request->getValue('contains');
     if (strlen($contains)) {
+
+      // See PHI720. If the standard "branch" field is provided, use it
+      // as the "pattern" argument to "git branch ..." to let callers test
+      // for reachability from a particular branch head.
+      $pattern = $request->getValue('branch');
+      if (strlen($pattern)) {
+        $pattern_argv = array($pattern);
+      } else {
+        $pattern_argv = array();
+      }
+
       // NOTE: We can't use DiffusionLowLevelGitRefQuery here because
       // `git for-each-ref` does not support `--contains`.
       if ($repository->isWorkingCopyBare()) {
         list($stdout) = $repository->execxLocalCommand(
-          'branch --verbose --no-abbrev --contains %s --',
-          $contains);
+          'branch --verbose --no-abbrev --contains %s -- %Ls',
+          $contains,
+          $pattern_argv);
         $ref_map = DiffusionGitBranch::parseLocalBranchOutput(
           $stdout);
       } else {
         list($stdout) = $repository->execxLocalCommand(
-          'branch -r --verbose --no-abbrev --contains %s --',
-          $contains);
+          'branch -r --verbose --no-abbrev --contains %s -- %Ls',
+          $contains,
+          $pattern_argv);
         $ref_map = DiffusionGitBranch::parseRemoteBranchOutput(
           $stdout,
           DiffusionGitBranch::DEFAULT_GIT_REMOTE);
diff --git a/src/applications/diffusion/controller/DiffusionCommitBranchesController.php b/src/applications/diffusion/controller/DiffusionCommitBranchesController.php
--- a/src/applications/diffusion/controller/DiffusionCommitBranchesController.php
+++ b/src/applications/diffusion/controller/DiffusionCommitBranchesController.php
@@ -22,6 +22,7 @@
         array(
           'contains' => $drequest->getCommit(),
           'limit' => $branch_limit + 1,
+          'branch' => null,
         )));
 
     $has_more_branches = (count($branches) > $branch_limit);