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