ref-filter: factor out the parsing of sorting atoms

ref-filter: factor out the parsing of sorting atoms

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>