Make two-commit diff easier to find

[wcooley]

I really love the two-commit diff added in PR #781 but the problem is that I keep having to refer to the PR to remember how to actually do it. If I do a secondary click to bring up the cursor-menu, one of the commits becomes unselected. It would be nice if there were an option in the menubar menu that that would show the diff or something like that. (I am realizing now that it's the same key as the "Diff with HEAD", which might be enough reminder for the future. I guess.)