Ability to diff against any parent (BB-6054)

Right now if I opt to see the diff for a commit that has multiple parents (i.e., a merge), the system chooses one of the parents, seemingly by random, to diff against.

Quite often it is not the parent I want, and I cannot find any way to diff against the other parent.

To make this easy, I think the list of parents that is currently shown in the top-right of the diff could be augmented with radio buttons to select which one to diff against.

Please don’t make me go through a more complex UI that presents me with all changesets to choose from. Such a UI may be useful to have so that you can diff between any arbitrary revisions, but for the common case of diffing against the parent, we should be able to do this with a single click.