qlog: Add ui to select parent to diff with.

[Gary van der Merwe]

Currently, qlog only shows the diff of the left hand parent. We should have some way to select which parent you want to diff with.

I'm not too sure how the ui should look. One option is to do the same as In bzr-gtk's viz, where you can click on a button next to the parent in the revision information. This currently won't work if you select multiple revisions (until we fix bug 412029, but then you will have to scroll down to the bottom of the revision info list.) This also does not give use a why to select which parent to diff for the file list. Maybe it should be a option button next to parent.

Another option would be to have a combo box above the file list. This does not show to the user that is is related to the diff button. Maybe the diff button can be move next to the combobox, or below the file list.

RFC