ref-filter: introduce remote_ref_atom_parser()

ref-filter: introduce remote_ref_atom_parser()

Introduce remote_ref_atom_parser() which will parse the '%(upstream)'
and '%(push)' atoms and store information into the 'used_atom'
structure based on the modifiers used along with the corresponding
atom.

Helped-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Karthik Nayak <Karthik.188@gmail.com>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>