Pass branch information to local Conduit calls in Diffusion

Summary: Fixes T3697. Currently, we don't pass "branch" implicitly, so, e.g., when viewing a branch you don't get the right commit hash when looking up the README.

Test Plan: Viewed a non-`master` branch with a README, no fatal. Poked around and couldn't find anything suspicious.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3697

Differential Revision: https://secure.phabricator.com/D6734