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