Summary:
Upstream 4718718ed358 requires a `readfast` method for manifestctx.
`hybridmanifestctx` does not have such method so let's add it.
Test Plan: Added a test.
Reviewers: durham, ttung, #sourcecontrol, rmcelroy
Reviewed By: rmcelroy
Subscribers: mjpieters
Differential Revision: https://phabricator.intern.facebook.com/D3955478
Signature: t1:3955478:1475582074:f80c9eb97bcd42846625727ef1da4bce3256c88c
