ref-filter: move ahead-behind bases into used_atom

verify_ref_format() parses a ref-filter format string and stores
recognized items in the static array "used_atom".  For
"ahead-behind:<committish>" it stores the committish part in a
string_list member "bases" of struct ref_format.

ref_sorting_options() also parses bare ref-filter format items and
stores stores recognized ones in "used_atom" as well.  The committish
parts go to a dummy struct ref_format in parse_sorting_atom(), though,
and are leaked and forgotten.

If verify_ref_format() is called before ref_sorting_options(), like in
git for-each-ref, then all works well if the sort key is included in the
format string.  If it isn't then sorting cannot work as the committishes
are missing.

If ref_sorting_options() is called first, like in git branch, then we
have the additional issue that if the sort key is included in the format
string then filter_ahead_behind() can't see its committish, will not
generate any results for it and thus it will be expanded to an empty
string.

Fix those issues by replacing the string_list with a field in used_atom
for storing the committish.  This way it can be shared for handling both
ref-filter format strings and sorting options in the same command.

Reported-by: Ross Goldberg <ross.goldberg@gmail.com>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/branch.c

@@ -473,7 +473,7 @@ static void print_ref_list(struct ref_filter *filter, struct ref_sorting *sortin
 	if (verify_ref_format(format))
 		die(_("unable to parse format string"));
 
-	filter_ahead_behind(the_repository, format, &array);
+	filter_ahead_behind(the_repository, &array);
 	ref_array_sort(sorting, &array);
 
 	if (column_active(colopts)) {