Use --no-ext-diff in arc land call to git diff

Summary:
Fixes T11435. This isn't a perfect solution since there's a little code duplication, but a perfect solution is probably a bit more involved.

See T11435 for some discussion. In particular, most `git diff` commands already get this flag via `ArcanistGitAPI->getDiffBaseOptions()`.

Test Plan: Will land this change.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11435

Differential Revision: https://secure.phabricator.com/D16375

src/land/ArcanistGitLandEngine.php

@@ -181,7 +181,10 @@ final class ArcanistGitLandEngine
           $this->getTargetFullRef()));
     }
 
-    list($changes) = $api->execxLocal('diff HEAD --');
+    // TODO: This could probably be cleaner by asking the API a question
+    // about working copy status instead of running a raw diff command. See
+    // discussion in T11435.
+    list($changes) = $api->execxLocal('diff --no-ext-diff HEAD --');
     $changes = trim($changes);
     if (!strlen($changes)) {
       throw new Exception(