[git] Override diff.submodule so git diff output is always parseable

Test Plan:
Removed a submodule with `diff.submodule` set to `log`, saw
`arc diff` error; with this change, it no longer does.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley

Maniphest Tasks: T10881

Differential Revision: https://secure.phabricator.com/D17327

src/repository/api/ArcanistGitAPI.php

@@ -446,6 +446,9 @@ final class ArcanistGitAPI extends ArcanistRepositoryAPI {
       // would ship up the binaries for 'arc patch' but display the textconv
       // output in the visual diff.
       '--no-textconv',
+      // Provide a standard view of submodule changes; the 'log' and 'diff'
+      // values do not parse by the diff parser.
+      '--submodule=short',
     );
     return implode(' ', $options);
   }