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.h

@@ -96,6 +96,11 @@ struct ref_format {
 #define OPT_MERGED(f, h) _OPT_MERGED_NO_MERGED("merged", f, h)
 #define OPT_NO_MERGED(f, h) _OPT_MERGED_NO_MERGED("no-merged", f, h)
 
+#define OPT_REF_SORT(var) \
+	OPT_CALLBACK_F(0, "sort", (var), \
+		       N_("key"), N_("field name to sort on"), \
+		       PARSE_OPT_NONEG, parse_opt_ref_sorting)
+
 /*
  * API for filtering a set of refs. Based on the type of refs the user
  * has requested, we iterate through those refs and apply filters