undo: restore the cursor position when a cut and paste are paired

When undoing/redoing an alternate spell check (of the entire file
or of a backwards marked region), put the cursor back where it was
when the spell check was invoked/finished.

This fixes https://savannah.gnu.org/bugs/?54466.

src/text.c

@@ -797,6 +797,8 @@ void do_undo(void)
 		break;
 	case COUPLE_BEGIN:
 		undidmsg = u->strdata;
+		if (u->xflags != MARK_WAS_SET)
+			goto_line_posx(u->lineno, u->begin);
 		break;
 	case COUPLE_END:
 		openfile->current_undo = openfile->current_undo->next;
@@ -965,6 +967,8 @@ void do_redo(void)
 		openfile->current_undo = u;
 		do_redo();
 		do_redo();
+		if (u->xflags != MARK_WAS_SET)
+			goto_line_posx(u->lineno, u->begin);
 		do_redo();
 		return;
 	case COUPLE_END: