ref-filter: add 'ref-filter.h'

ref-filter: add 'ref-filter.h'

This is step one of creating a common library for 'for-each-ref',
'branch -l' and 'tag -l'. This creates a header file with the
functions and data structures that ref-filter will provide.
We move the data structures created in for-each-ref to this header
file.

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