undo: start renumbering from the head of the paste, not from its tail

Because when undoing a paste at the end of the buffer, the tail line
probably does not exist any more.

This fixes https://savannah.gnu.org/bugs/?48222.

(It would have been the proper fix for
https://savannah.gnu.org/bugs/?44488.)

src/text.c

@@ -751,7 +751,7 @@ void do_undo(void)
     case PASTE:
 	undidmsg = _("text uncut");
 	undo_paste(u);
-	f = fsfromline(u->lineno);
+	f = fsfromline(u->mark_begin_lineno);
 	break;
     case ENTER:
 	if (f->next == NULL) {