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://secure.phabricator.com/D15499