parse_opt_ref_sorting: always use with NONEG flag

The "--sort" parameter of for-each-ref, etc, does not handle negation,
and instead returns an error to the parse-options code. But neither
piece of code prints anything for the user, which may leave them
confused:

  $ git for-each-ref --no-sort
  $ echo $?
  129

As the comment in the callback function notes, this probably should
clear the list, which would make it consistent with other list-like
options (i.e., anything that uses OPT_STRING_LIST currently).
Unfortunately that's a bit tricky due to the way the ref-filter code
works. But in the meantime, let's at least make the error a little less
confusing:

  - switch to using PARSE_OPT_NONEG in the option definition, which will
    cause the options code to produce a useful message

  - since this was cut-and-pasted to four different spots, let's define
    a single OPT_REF_SORT() macro that we can use everywhere

  - the callback can use BUG_ON_OPT_NEG() to make sure the correct flags
    are used (incidentally, this also satisfies -Wunused-parameters,
    since we're now looking at "unset")

  - expand the comment into a NEEDSWORK to make it clear that the
    direction is right, but the details need to be worked out

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ref-filter.c

@@ -2337,8 +2337,13 @@ void parse_ref_sorting(struct ref_sorting **sorting_tail, const char *arg)
 
 int parse_opt_ref_sorting(const struct option *opt, const char *arg, int unset)
 {
-	if (!arg) /* should --no-sort void the list ? */
-		return -1;
+	/*
+	 * NEEDSWORK: We should probably clear the list in this case, but we've
+	 * already munged the global used_atoms list, which would need to be
+	 * undone.
+	 */
+	BUG_ON_OPT_NEG(unset);
+
 	parse_ref_sorting(opt->value, arg);
 	return 0;
 }