Added a conduit method for getting all the diffs for a revision.

Summary:
Loading all diffs for a differential revision is needed by at least
perflab.

Test Plan:
Created a simple script that queried the conduit and made sure that
it returned correct values.

Reviewed By: jungejason
Reviewers: jungejason
CC: epriestley, jungejason
Differential Revision: 155