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

pkg/integration/tests/ui/range_select.go

@@ -159,19 +159,11 @@ var RangeSelect = NewIntegrationTest(NewIntegrationTestArgs{
 				SelectedLines(
 					Contains("line 1"),
 				).
-				Press(keys.Universal.RangeSelectDown)
-			if lineIdxOfFirstItem == 6 {
-				v.SelectedLines(
-					// bug: it moved to line 2 instead of selecting both line 1 and line 2
-					Contains("line 2"),
-				)
-			} else {
-				// works correctly in list views though
-				v.SelectedLines(
+				Press(keys.Universal.RangeSelectDown).
+				SelectedLines(
 					Contains("line 1"),
 					Contains("line 2"),
 				)
-			}
 		}
 
 		assertRangeSelectBehaviour(t.Views().Commits().Focus(), t.Views().Branches(), 0)