Merge branch 'zh/ref-filter-atom-type'

The code to handle the "--format" option in "for-each-ref" and
friends made too many string comparisons on %(atom)s used in the
format string, which has been corrected by converting them into
enum when the format string is parsed.

* zh/ref-filter-atom-type:
  ref-filter: introduce enum atom_type
  ref-filter: add objectsize to used_atom