Set main views to underline hyperlinks only on mouse hover

Note that this doesn't only affect the diff views, which are the ones where we
want this, but also the status view, where the plan was to keep them underlined
always for better discoverability. We could make this configurable dynamically
(by adding another flag to ViewUpdateOpts), but actually I think it's fine this
way.

pkg/gui/views.go

@@ -117,6 +117,7 @@ func (gui *Gui) createAllViews() error {
 		view.Title = gui.c.Tr.DiffTitle
 		view.Wrap = true
 		view.IgnoreCarriageReturns = true
+		view.UnderlineHyperLinksOnlyOnHover = true
 	}
 
 	gui.Views.Staging.Title = gui.c.Tr.UnstagedChanges