branch.c: use 'ref-filter' data structures

Make 'branch.c' use 'ref-filter' data structures and make changes to
support the new data structures. This is a part of the process of
porting 'branch.c' to use 'ref-filter' APIs.

This is a temporary step before porting 'branch.c' to use 'ref-filter'
completely. As this is a temporary step, most of the code introduced
here will be removed when 'branch.c' is ported over to use
'ref-filter' APIs.

Mentored-by: Christian Couder <[email protected]>
Mentored-by: Matthieu Moy <[email protected]>
Signed-off-by: Karthik Nayak <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: KarthikNayak