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/for-each-ref.c

@@ -37,8 +37,7 @@ int cmd_for_each_ref(int argc, const char **argv, const char *prefix)
 		OPT_INTEGER( 0 , "count", &maxcount, N_("show only <n> matched refs")),
 		OPT_STRING(  0 , "format", &format.format, N_("format"), N_("format to use for the output")),
 		OPT__COLOR(&format.use_color, N_("respect format colors")),
-		OPT_CALLBACK(0 , "sort", sorting_tail, N_("key"),
-			    N_("field name to sort on"), &parse_opt_ref_sorting),
+		OPT_REF_SORT(sorting_tail),
 		OPT_CALLBACK(0, "points-at", &filter.points_at,
 			     N_("object"), N_("print only refs which points at the given object"),
 			     parse_opt_object_name),