diff --git a/src/workflow/ArcanistDiffWorkflow.php b/src/workflow/ArcanistDiffWorkflow.php
--- a/src/workflow/ArcanistDiffWorkflow.php
+++ b/src/workflow/ArcanistDiffWorkflow.php
@@ -609,6 +609,12 @@
     if ($repository_api->supportsCommitRanges()) {
       $this->parseBaseCommitArgument($this->getArgument('paths'));
     }
+
+    $head_commit = $this->getArgument('head');
+    if ($head_commit !== null) {
+      $repository_api->setHeadCommit($head_commit);
+    }
+
   }
 
   private function runDiffSetupBasics() {