That "refs/bisect/" has to be handled specially when filling the
ref_cache for loose references is a peculiarity of the files backend,
and the ref-cache code shouldn't need to know about it. So move this
code to the callback function, `loose_fill_ref_dir()`.
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu> Signed-off-by: Junio C Hamano <gitster@pobox.com>