cut: concentrate the logic for clearing the cutbuffer mostly in one place

It should not be the task of the undo code to take care that the
cutbuffer will be cleared at the right moments.

src/cut.c

@@ -125,12 +125,12 @@ void do_cut_text(bool copy_text, bool cut_till_eof)
 #endif
 	size_t was_totsize = openfile->totsize;
 
-	/* If a chain of cuts was broken, empty the cutbuffer. */
-	if (!keep_cutbuffer) {
+	/* If cuts were not continuous, or when cutting a region, clear the slate. */
+	if (!keep_cutbuffer || openfile->mark || cut_till_eof) {
 		free_filestruct(cutbuffer);
 		cutbuffer = NULL;
-		/* Indicate that future cuts should add to the cutbuffer. */
-		keep_cutbuffer = TRUE;
+		/* After a line cut, future line cuts should add to the cutbuffer. */
+		keep_cutbuffer = !openfile->mark && !cut_till_eof;
 	}
 
 #ifndef NANO_TINY

src/nano.c

@@ -1759,7 +1759,6 @@ int do_input(bool allow_funcs)
 		if (shortcut->func == do_cut_text_void
 #ifndef NANO_TINY
 				|| shortcut->func == do_copy_text
-				|| shortcut->func == do_cut_till_eof
 #endif
 				)
 			retain_cuts = TRUE;

src/text.c

@@ -1395,10 +1395,8 @@ void add_undo(undo_type action)
 		break;
 #endif
 	case CUT_TO_EOF:
-		cutbuffer_reset();
 		break;
 	case CUT:
-		cutbuffer_reset();
 		if (openfile->mark) {
 			u->mark_begin_lineno = openfile->mark->lineno;
 			u->mark_begin_x = openfile->mark_x;