ref-filter: move ref_sorting flags to a bitfield

Change the reverse/ignore_case/version sort flags in the ref_sorting
struct into a bitfield. Having three of them was already a bit
unwieldy, but it would be even more so if another flag needed a
function like ref_sorting_icase_all() introduced in
76f9e569ad (ref-filter: apply --ignore-case to all sorting keys,
2020-05-03).

A follow-up change will introduce such a flag, so let's move this over
to a bitfield. Instead of using the usual '#define' pattern I'm using
the "enum" pattern from builtin/rebase.c's b4c8eb024a (builtin
rebase: support --quiet, 2018-09-04).

Perhaps there's a more idiomatic way of doing the "for each in list
amend mask" pattern than this "mask/on" variable combo. This function
doesn't allow us to e.g. do any arbitrary changes to the bitfield for
multiple flags, but I think in this case that's fine. The common case
is that we're calling this with a list of one.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/tag.c

@@ -485,7 +485,7 @@ int cmd_tag(int argc, const char **argv, const char *prefix)
 	}
 	if (!sorting)
 		sorting = ref_default_sorting();
-	ref_sorting_icase_all(sorting, icase);
+	ref_sorting_set_sort_flags_all(sorting, REF_SORTING_ICASE, icase);
 	filter.ignore_case = icase;
 	if (cmdmode == 'l') {
 		int ret;