fail arc diff if second lfs push errors

Summary:
We are having issues where people run out of file descriptors and the first `git push` will succeed, but the
second one will not. We'd like the diff to not be created in this case as it leads to weird behavior like our tests
running against 0 changed files.

Test Plan: none

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D21471

src/workflow/ArcanistDiffWorkflow.php

@@ -2741,6 +2741,17 @@ EOTEXT
         'git push -- %s %s',
         $staging_uri,
         $ref);
+
+      if ($err) {
+        $this->writeWarn(
+          pht('STAGING FAILED'),
+          pht('Unable to push lfs changes to the staging area.'));
+
+        throw new ArcanistUsageException(
+          pht(
+            'Failed to push lfs changes to staging area. Correct the issue, '.
+            'or use --skip-staging to skip this step.'));
+      }
     }
 
     return $refs;