tweaks: really don't bother renumbering the lines in the cutbuffer

The lines from the cutbuffer will be renumbered when it matters: when
they get pasted.

The only place that used the numbering of (a copy of) the cutbuffer
(the updating of an undo item) already iterates through its lines.
Just add a counter there instead of making use of the line numbers.

src/nano.c

@@ -317,10 +317,7 @@ void extract(linestruct *top, size_t top_x, linestruct *bot, size_t bot_x)
 	if (cutbuffer == NULL) {
 		cutbuffer = openfile->filetop;
 		cutbottom = openfile->filebot;
-		renumber_from(cutbuffer);
 	} else {
-		linestruct *was_bottom = cutbottom;
-
 		/* Tack the data of the first line of the text onto the data of
 		 * the last line in the given buffer. */
 		cutbottom->data = charealloc(cutbottom->data,
@@ -339,8 +336,6 @@ void extract(linestruct *top, size_t top_x, linestruct *bot, size_t bot_x)
 			cutbottom->next->prev = cutbottom;
 			cutbottom = openfile->filebot;
 		}
-
-		renumber_from(was_bottom);
 	}
 
 	/* Since the text has now been saved, remove it from the file buffer. */

src/text.c

@@ -1371,13 +1371,14 @@ void update_undo(undo_type action)
 				u->xflags |= WAS_MARKED_FORWARD;
 		} else {
 			linestruct *bottomline = u->cutbuffer;
+			size_t count = 0;
 
 			/* Find the end of the cut for the undo/redo, using our copy. */
-			while (bottomline->next != NULL)
+			while (bottomline->next != NULL) {
 				bottomline = bottomline->next;
-
-			u->lineno = u->mark_begin_lineno + bottomline->lineno -
-										u->cutbuffer->lineno;
+				count++;
+			}
+			u->lineno = u->mark_begin_lineno + count;
 			if (ISSET(CUT_FROM_CURSOR) || u->type == CUT_TO_EOF) {
 				u->begin = strlen(bottomline->data);
 				if (u->lineno == u->mark_begin_lineno)