Improve handling of bad branches in Diffusion

Summary:
Fixes T7972.

  - Trap the RefNotFound error which may occur in `getAlternatives()`.
  - Improve error handling in Mercurial.

Test Plan: {F387611}

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7972

Differential Revision: https://secure.phabricator.com/D12590

src/applications/diffusion/query/lowlevel/DiffusionLowLevelResolveRefsQuery.php

@@ -204,6 +204,10 @@ final class DiffusionLowLevelResolveRefsQuery
           // unclear how to best do that. For now, treat it as a miss instead.
           continue;
         }
+        if (preg_match('/unknown revision/', $ex->getStdErr())) {
+          // No matches for this ref.
+          continue;
+        }
         throw $ex;
       }
 

src/applications/diffusion/request/DiffusionRequest.php

@@ -755,7 +755,12 @@ abstract class DiffusionRequest {
 
   public function getRefAlternatives() {
     // Make sure we've resolved the reference into a stable commit first.
-    $this->getStableCommit();
+    try {
+      $this->getStableCommit();
+    } catch (DiffusionRefNotFoundException $ex) {
+      // If we have a bad reference, just return the empty set of
+      // alternatives.
+    }
     return $this->refAlternatives;
   }