ref-filter.c: parse trailers arguments with %(contents) atom

ref-filter.c: parse trailers arguments with %(contents) atom

The %(contents) atom takes a contents "field" as its argument. Since
"trailers" is one of those fields, extend contents_atom_parser to parse
"trailers"'s arguments when used through "%(contents)", like:

  %(contents:trailers:unfold,only)

A caveat: trailers_atom_parser expects NULL when no arguments are given
(see: `parse_ref_filter_atom`). This is because string_list_split (given
a maxsplit of -1) returns a 1-ary string_list* containing the given
string if the delimiter could not be found using `strchr`.

To simulate this behavior without teaching trailers_atom_parser to
accept strings with length zero, conditionally pass NULL to
trailers_atom_parser if the arguments portion of the argument to
%(contents) is empty.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>