Diff any two nodes in history view

[dhirschfeld]

Moved over from #245.

As it says in the title, it would be very convenient to be able to see the diff between any two points selected in the history view.

[jaipreet-s]

Hi @dhirschfeld , I'm consolidating discussions around visual diff into #317. Thanks

[fcollonval]

Now that #900 is in this should be easy to implement by defining a context menu in the history tab to select to node and call the gitShowDiff commands with the proper DiffModel

[fcollonval]

Fixed by #1108 and #1117