Improve Undo & Redo actions return values

Return false if there is nothing to undo/redo.

This also fixes false "Undid action" and "Redid actions" infobar
messages in the case when no action was actually undone or redone.

internal/action/actions.go

@@ -1163,7 +1163,9 @@ func (h *BufPane) DiffPrevious() bool {
 
 // Undo undoes the last action
 func (h *BufPane) Undo() bool {
-	h.Buf.Undo()
+	if !h.Buf.Undo() {
+		return false
+	}
 	InfoBar.Message("Undid action")
 	h.Relocate()
 	return true
@@ -1171,7 +1173,9 @@ func (h *BufPane) Undo() bool {
 
 // Redo redoes the last action
 func (h *BufPane) Redo() bool {
-	h.Buf.Redo()
+	if !h.Buf.Redo() {
+		return false
+	}
 	InfoBar.Message("Redid action")
 	h.Relocate()
 	return true

internal/buffer/eventhandler.go

@@ -253,11 +253,11 @@ func (eh *EventHandler) Execute(t *TextEvent) {
 	ExecuteTextEvent(t, eh.buf)
 }
 
-// Undo the first event in the undo stack
-func (eh *EventHandler) Undo() {
+// Undo the first event in the undo stack. Returns false if the stack is empty.
+func (eh *EventHandler) Undo() bool {
 	t := eh.UndoStack.Peek()
 	if t == nil {
-		return
+		return false
 	}
 
 	startTime := t.Time.UnixNano() / int64(time.Millisecond)
@@ -266,15 +266,16 @@ func (eh *EventHandler) Undo() {
 	for {
 		t = eh.UndoStack.Peek()
 		if t == nil {
-			return
+			break
 		}
 
 		if t.Time.UnixNano()/int64(time.Millisecond) < endTime {
-			return
+			break
 		}
 
 		eh.UndoOneEvent()
 	}
+	return true
 }
 
 // UndoOneEvent undoes one event
@@ -303,11 +304,11 @@ func (eh *EventHandler) UndoOneEvent() {
 	eh.RedoStack.Push(t)
 }
 
-// Redo the first event in the redo stack
-func (eh *EventHandler) Redo() {
+// Redo the first event in the redo stack. Returns false if the stack is empty.
+func (eh *EventHandler) Redo() bool {
 	t := eh.RedoStack.Peek()
 	if t == nil {
-		return
+		return false
 	}
 
 	startTime := t.Time.UnixNano() / int64(time.Millisecond)
@@ -316,15 +317,16 @@ func (eh *EventHandler) Redo() {
 	for {
 		t = eh.RedoStack.Peek()
 		if t == nil {
-			return
+			break
 		}
 
 		if t.Time.UnixNano()/int64(time.Millisecond) > endTime {
-			return
+			break
 		}
 
 		eh.RedoOneEvent()
 	}
+	return true
 }
 
 // RedoOneEvent redoes one event