add a getrevision conduit method

Summary:
Add a method to get the information about a revision, including its full
list of diffs.

We could add an option to just return the diff IDs if we wanted. For my
use case, I need the full set of information for each diff, so fewer
round trips is better. This is also how the old json.php page used to
work.

Test Plan:
Tuomas tested it in his sandbox.

Reviewed By: tuomaspelkonen
Reviewers: jungejason, tuomaspelkonen, epriestley
CC: tuomaspelkonen
Differential Revision: 156