Merge branch 'ot/cat-batch-format' into pu

Merge branch 'ot/cat-batch-format' into pu

Teach "cat-file --batch" to reuse the formatting machinery shared
by for-each-ref, branch --list, and tag --list.

Allocates flex-array on stack, etc.
cf. <58b2bdcd-d621-fd21-ab4d-6a9478319b19@ramsayjones.plus.com>

* ot/cat-batch-format: (23 commits)
  cat-file: update of docs
  cat-file: tests for new atoms added
  for-each-ref: tests for new atoms added
  ref-filter: unifying formatting of cat-file opts
  ref-filter: make populate_value() internal again
  cat-file: reuse printing logic from ref-filter
  ref-filter: make valid_atom general again
  ref-filter: make cat_file_info independent
  cat-file: move skip_object_info into ref-filter
  ref_filter: add is_atom_used function
  ref-filter: get rid of mark_atom_in_object_info()
  cat-file: start reusing populate_value()
  ref-filter: rename field in ref_array_item stuct
  ref-filter: make populate_value() global
  cat-file: start use ref_array_item struct
  ref-filter: reuse parse_ref_filter_atom()
  cat-file: start migrating formatting to ref-filter
  cat-file: split expand_atom() into 2 functions
  cat-file: move struct expand_data into ref-filter
  ref-filter: make valid_atom as function parameter
  ...