Code clean-ups.

* jt/unify-object-info:
  sha1_file: refactor has_sha1_file_with_flags
  sha1_file: do not access pack if unneeded
  sha1_file: teach sha1_object_info_extended more flags
  sha1_file: refactor read_object
  sha1_file: move delta base cache code up
  sha1_file: rename LOOKUP_REPLACE_OBJECT
  sha1_file: rename LOOKUP_UNKNOWN_OBJECT
  sha1_file: teach packed_object_info about typename