ref-filter: provide a function for parsing sort options

ref-filter: provide a function for parsing sort options

The ref-filter module currently provides a callback suitable
for parsing command-line --sort options. But since git-tag
also supports the tag.sort config option, it needs a
function whose implementation is quite similar, but with a
slightly different interface. The end result is that
builtin/tag.c has a copy-paste of parse_opt_ref_sorting().

Instead, let's provide a function to parse an arbitrary
sort string, which we can then trivially wrap to make the
parse_opt variant.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>