Merge branch 'mh/ref-api-lazy-loose'

Merge branch 'mh/ref-api-lazy-loose'

Refs API is updated to lazily read sub-hierarchies of refs/ namespace,
so that we do not have to grab everything from the filesystem when we
are only interested in listing branches, for example.

By Michael Haggerty (17) and Junio C Hamano (1)
* mh/ref-api-lazy-loose:
  refs: fix find_containing_dir() regression
  refs: read loose references lazily
  read_loose_refs(): eliminate ref_cache argument
  struct ref_dir: store a reference to the enclosing ref_cache
  search_for_subdir(): return (ref_dir *) instead of (ref_entry *)
  get_ref_dir(): add function for getting a ref_dir from a ref_entry
  read_loose_refs(): rename function from get_ref_dir()
  refs: wrap top-level ref_dirs in ref_entries
  find_containing_dir(): use strbuf in implementation of this function
  bisect: copy filename string obtained from git_path()
  do_for_each_reflog(): use a strbuf to hold logfile name
  do_for_each_reflog(): return early on error
  get_ref_dir(): take the containing directory as argument
  refs.c: extract function search_for_subdir()
  get_ref_dir(): require that the dirname argument ends in '/'
  get_ref_dir(): rename "base" parameter to "dirname"
  get_ref_dir(): use a strbuf to hold refname
  get_ref_dir(): return early if directory cannot be read