Fix pressing shift-down after clicking in diff view

When clicking in a single-file diff view to enter staging (or custom patch
editing, when coming from the commit files panel), and then pressing shift-down
or shift-up to select a range, it would move the selected line rather than
creating a range. Only on the next press would it start to select a range from
there.

This is very similar to the fix we made for pressing escape in 0e4d266a52.

pkg/gui/patch_exploring/state.go

@@ -94,7 +94,7 @@ func (s *State) ToggleStickySelectRange() {
 }
 
 func (s *State) ToggleSelectRange(sticky bool) {
-	if s.selectMode == RANGE {
+	if s.SelectingRange() {
 		s.selectMode = LINE
 	} else {
 		s.selectMode = RANGE