A few atoms like %(objecttype) and %(objectsize) in the format
specifier of "for-each-ref --format=<format>" can be filled without
getting the full contents of the object, but just with the object
header.  These cases have been optimized by calling
oid_object_info() API (instead of reading and inspecting the data).

* ot/ref-filter-object-info:
  ref-filter: use oid_object_info() to get object
  ref-filter: merge get_obj and get_object
  ref-filter: initialize eaten variable
  ref-filter: fill empty fields with empty values
  ref-filter: add info_source to valid_atom