pack-bitmap.c: make object filtering functions generic

pack-bitmap.c: make object filtering functions generic

In 4f3bd5606a (pack-bitmap: implement BLOB_NONE filtering, 2020-02-14),
filtering support for bitmaps was added for the 'LOFC_BLOB_NONE' filter.

In the future, we would like to add support for filters that behave as
if they exclude a certain type of object, for e.g., the tree depth
filter with depth 0.

To prepare for this, make some of the functions used for filtering more
generic, such as 'find_tip_blobs' and 'filter_bitmap_blob_none' so that
they can work over arbitrary object types.

To that end, create 'find_tip_objects' and
'filter_bitmap_exclude_type', and redefine the aforementioned functions
in terms of those.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>