ref-filter: introduce match_atom_name()

ref-filter: introduce match_atom_name()

Introduce match_atom_name() which helps in checking if a particular
atom is the atom we're looking for and if it has a value attached to
it or not.

Use it instead of starts_with() for checking the value of %(color:...)
atom. Write a test for the same.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Matthieu Moy <matthieu.moy@grenoble-inp.fr>
Thanks-to: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>