ref-filter: introduce contents_atom_parser()

ref-filter: introduce contents_atom_parser()

Introduce contents_atom_parser() which will parse the '%(contents)'
atom and store information into the 'used_atom' structure based on the
modifiers used along with the atom. Also introduce body_atom_parser()
and subject_atom_parser() for parsing atoms '%(body)' and '%(subject)'
respectively.

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>