Make SelectedLine/SelectedLineIdx work in staging/stagingSecondary views

While we try to keep the view's cursor position in sync with the context state's
selectedLineIdx (at least when pressing up or down), there are enough situations
where the two run out of sync; for example when initially opening the view, or
after staging a hunk, or when scrolling the view using the wheel. While it would
be possible to fix these situations to keep them always in sync, it doesn't seem
worth it, because the view's cursor position isn't really used for anything
else. So we rather special-case the SelectedLine/SelectedLineIdx functions of
ViewDriver to query the context state's selectedLineIdx directly if it is a
patch explorer context.

pkg/integration/components/test_test.go

@@ -30,6 +30,10 @@ func (self *fakeGuiDriver) CurrentContext() types.Context {
 	return nil
 }
 
+func (self *fakeGuiDriver) ContextForView(viewName string) types.Context {
+	return nil
+}
+
 func (self *fakeGuiDriver) Fail(message string) {
 	self.failureMessage = message
 }