R1:a24f001b08ef

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.