list-objects-filter: encapsulate filter components

list-objects-filter: encapsulate filter components

Encapsulate filter_fn, filter_free_fn, and filter_data into their own
opaque struct.

Due to opaqueness, filter_fn and filter_free_fn can no longer be
accessed directly by users. Currently, all usages of filter_fn are
guarded by a necessary check:

	(obj->flags & NOT_USER_GIVEN) && filter_fn

Take the opportunity to include this check into the new function
list_objects_filter__filter_object(), so that we no longer need to write
this check at every caller of the filter function.

Also, the init functions in list-objects-filter.c no longer need to
confusingly return the filter constituents in various places (filter_fn
and filter_free_fn as out parameters, and filter_data as the function's
return value); they can just initialize the "struct filter" passed in.

Helped-by: Jeff Hostetler <git@jeffhostetler.com>
Helped-by: Jonathan Tan <jonathantanmy@google.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Matthew DeVore <matvore@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>