Don't sort ref_list too early

Don't sort ref_list too early

get_ref_dir is called recursively for subdirectories, which means that
we were calling sort_ref_list for each directory of refs instead of
once for all the refs.  This is a massive wast of processing, so now
just call sort_ref_list on the result of the top-level get_ref_dir, so
that the sort is only done once.

In the common case of only a few different directories of refs the
difference isn't very noticable, but it becomes very noticeable when
you have a large number of direcotries containing refs (e.g. as
created by Gerrit).

Reported by Martin Fick.

Signed-off-by: Julian Phillips <julian@quantumfyre.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>