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>

builtin/tag.c

@@ -412,8 +412,7 @@ int cmd_tag(int argc, const char **argv, const char *prefix)
 		OPT_WITHOUT(&filter.no_commit, N_("print only tags that don't contain the commit")),
 		OPT_MERGED(&filter, N_("print only tags that are merged")),
 		OPT_NO_MERGED(&filter, N_("print only tags that are not merged")),
-		OPT_CALLBACK(0 , "sort", sorting_tail, N_("key"),
-			     N_("field name to sort on"), &parse_opt_ref_sorting),
+		OPT_REF_SORT(sorting_tail),
 		{
 			OPTION_CALLBACK, 0, "points-at", &filter.points_at, N_("object"),
 			N_("print only tags of the object"), PARSE_OPT_LASTARG_DEFAULT,