Show merged changes in commit views for merges

Summary:
When a commit is a merge, show what it merged.

Also fix some bugs:

• Mercurial queries may contain ":", but mercurial rev ranges may also contain ":". A rev range with a branch that has a ":" in it is ambigiuous, e.g. branch "a:b" might appear in a rev range like "a:b:0", which can not be parsed. Use stable commit names instead.
• Mercurial stable commit name implementation was broken, fix it.
• Extend DiffusionHistoryQuery from DiffusionQuery to share code.
• Fix a bug where Mercurial's main browse list would not show the most recent commit if it was a merge commit.

Test Plan: Generated a bunch of mercurial/git merge commits and looked at them, they seemed to accurately represent the repository state.

Reviewers: btrahan, Makinde

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T961

Differential Revision: https://secure.phabricator.com/D2005