R1:76f07ec80bde

Only require view permissions for read-only Git LFS requests

Summary: Ref T7789. Implement proper detection for read-only requests. Previously, we assumed every request was read/write and required lots of permissions, but we don't need "Can Push" permission if you're only cloning/fetching/pulling.

Test Plan:
- Set push policy to "no one".
- Fetched, got clean data out of LFS.
- Tried to push, got useful error.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7789

Differential Revision: https…