Merge branch 'mh/ref-api'

Allows walking only a sub-hierarchy in refs/*.

By Michael Haggerty
* mh/ref-api:
  do_for_each_ref(): only iterate over the subtree that was requested
  refs: store references hierarchically
  sort_ref_dir(): simplify logic
  refs.c: rename ref_array -> ref_dir
  struct ref_entry: nest the value part in a union
  check_refname_component(): return 0 for zero-length components
  free_ref_entry(): new function
  names_conflict(): simplify implementation
  repack_without_ref(): reimplement using do_for_each_ref_in_array()
  do_for_each_ref_in_arrays(): new function
  do_for_each_ref_in_array(): new function
  refs: manage current_ref within do_one_ref()
  refs.c: reorder definitions more logically