Merge branch 'tb/bitmap-walk-with-tree-zero-filter'

The object walk with object filter "--filter=tree:0" can now take
advantage of the pack bitmap when available.

* tb/bitmap-walk-with-tree-zero-filter:
  pack-bitmap: pass object filter to fill-in traversal
  pack-bitmap.c: support 'tree:0' filtering
  pack-bitmap.c: make object filtering functions generic
  list-objects-filter: treat NULL filter_options as "disabled"