Compare two branches

Summary:
This shows the commits list only (Actual git diff will show up at a later date).
The inputs are left as text-fields, to allow the form to accept anything that can be resolved. The form is GET, to allow sharing URIs.

The conduit method response array is compatible with that of diffusion.historyquery, to make it easy to build
the "history" table.

The hardest part here was, of course, Naming. I think "from" and "onto" are unconfusing, and I'm fairly confident that the "to merge"
instructions are in sync with the actual content of the page.

Test Plan: Look at several "compare" views, with various values of "from" and "onto".

Reviewers: #blessed_reviewers!, epriestley

Subscribers: caov297, 20after4, Sam2304, reardencode, baileyb, chad, Korvin

Maniphest Tasks: T929

Differential Revision: https://secure.phabricator.com/D15330