R1:d28eb759d622

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…