We parse sort strings as single formatting atoms, and just
build on parse_ref_filter_atom(). Let's pull this idea into
its own function, since it's about to get a little more
complex. As a bonus, we can give the function a slightly
more natural interface, since our single atoms are in their
own strings.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>