Introduce filter_refs() which will act as an API for filtering
a set of refs. Based on the type of refs the user has requested,
we iterate through those refs and apply filters as per the
given ref_filter structure and finally store the filtered refs
in the ref_array structure.

Currently this will wrap around ref_filter_handler(). Hence,
ref_filter_handler is made file scope static.

As users of this API will no longer send a ref_filter_cbdata
structure directly, we make the elements of ref_filter_cbdata
pointers. We can now use the information given by the users
to obtain our own ref_filter_cbdata structure. Changes are made to
support the change in ref_filter_cbdata structure.

Make 'for-each-ref' use this API.
Helped-by: NJunio C Hamano <gitster@pobox.com>
Mentored-by: NChristian Couder <christian.couder@gmail.com>
Mentored-by: NMatthieu Moy <matthieu.moy@grenoble-inp.fr>
Signed-off-by: NKarthik Nayak <karthik.188@gmail.com>
Reviewed-by: NMatthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>