In preparation for implementing narrow/partial clone, the object
walking machinery has been taught a way to tell it to "filter" some
objects from enumeration.

* jh/object-filtering:
  rev-list: support --no-filter argument
  list-objects-filter-options: support --no-filter
  list-objects-filter-options: fix 'keword' typo in comment
  pack-objects: add list-objects filtering
  rev-list: add list-objects filtering support
  list-objects: filter objects in traverse_commit_list
  oidset: add iterator methods to oidset
  oidmap: add oidmap iterator methods
  dir: allow exclusions from blob in addition to file