Support pushing data into Git LFS

Summary:
Ref T7789. Ref T10604. This implements the `upload` action, which streams file data into Files.

This makes Git LFS actually work, at least roughly.

Test Plan:
  - Tracked files in an LFS repository.
  - Pushed LFS data (`git lfs track '*.png'; git add something.png; git commit -m ...; git push`).
  - Pulled LFS data (`git checkout master^; rm -rf .git/lfs; git checkout master; open something.png`).
  - Verified LFS refs show up in the gitlfsref table.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7789, T10604

Differential Revision: https://secure.phabricator.com/D15492

src/applications/files/uploadsource/PhabricatorFileUploadSource.php

@@ -72,7 +72,9 @@ abstract class PhabricatorFileUploadSource
       $data->rewind();
       $this->didRewind = true;
     } else {
-      $data->next();
+      if ($data->valid()) {
+        $data->next();
+      }
     }
 
     if (!$data->valid()) {