R1:01844087cd85

Added a conduit method for querying revision feedback.

Summary:
Push tools needs information about differential revision comments. Added
a method that returns the needed information.

Test Plan:
Tested that /intern/push/merges shows correct information.

Reviewed By: jungejason
Reviewers: jungejason
CC: epriestley, jungejason
Differential Revision: 152