Add callback data to for_each_ref() family.

Add callback data to for_each_ref() family.

This is a long overdue fix to the API for for_each_ref() family
of functions.  It allows the callers to specify a callback data
pointer, so that the caller does not have to use static
variables to communicate with the callback funciton.

The updated for_each_ref() family takes a function of type

	int (*fn)(const char *, const unsigned char *, void *)

and a void pointer as parameters, and calls the function with
the name of the ref and its SHA-1 with the caller-supplied void
pointer as parameters.

The commit updates two callers, builtin-name-rev.c and
builtin-pack-refs.c as an example.

Signed-off-by: Junio C Hamano <junkio@cox.net>