Clear range select upon pressing 'escape'

This is the highest priority of the escape actions because it's the thing you're
most likely to want to do upon hitting escape if you have a range selected.

Applying this to the staging/patch-building views is tricky: if we want this logic
for when a range of lines is selected, we'll also need to apply it when a hunk
is selected too. I still think it's worth it though: I've often accidentally
escaped from the staging view when trying to cancel a range selection.

pkg/gui/controllers/patch_building_controller.go

@@ -154,5 +154,13 @@ func (self *PatchBuildingController) toggleSelection() error {
 }
 
 func (self *PatchBuildingController) Escape() error {
+	context := self.c.Contexts().CustomPatchBuilder
+	state := context.GetState()
+
+	if state.SelectingRange() || state.SelectingHunk() {
+		state.SetLineSelectMode()
+		return self.c.PostRefreshUpdate(context)
+	}
+
 	return self.c.Helpers().PatchBuilding.Escape()
 }