get_ref_dir(): take the containing directory as argument

get_ref_dir(): take the containing directory as argument

Previously, the "dir" argument to get_ref_dir() was a pointer to the
top-level ref_dir.  Change the function to expect a pointer to the
ref_dir corresponding to dirname.  This allows entries to be added
directly to dir, without having to recurse through the reference trie
each time (i.e., we can use add_entry_to_dir() instead of add_ref()).

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>