R1:3bacba7e9fe6

Show parent commits in Diffusion

Summary: Show parent commit information to make it easier to understand merges.

Test Plan: Looked at commits in SVN, hg, git.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T961

Differential Revision: https://secure.phabricator.com/D2021