ArcanistGitAPI: document a git ls-files bug

When a submodule is ignored (ignore=all in .gitconfig),

  $ git ls-files -m

fails to exclude the submodule from the listing. Other commands like

  $ git diff-index --name-only HEAD

exclude it just fine.

See: <https://github.com/facebook/arcanist/pull/120>

Reviewed by: epriestley

src/repository/api/ArcanistGitAPI.php

@@ -474,6 +474,8 @@ final class ArcanistGitAPI extends ArcanistRepositoryAPI {
       ));
 
     // Unstaged changes
+    // TODO: This doesn't exclude ignored submodules.
+    // Upstream bug: http://thread.gmane.org/gmane.comp.version-control.git/238173
     $unstaged_future = $this->buildLocalFuture(
       array(
         'ls-files -m',