refs.c: remove_empty_directories can take a strbuf

refs.c: remove_empty_directories can take a strbuf

The first thing we do in this function is copy the input
into a strbuf. Of the 4 callers, 3 of them already have a
strbuf we could use. Let's just take the strbuf, and convert
the remaining caller to use a strbuf, rather than a raw
git_path. This is safer, anyway, as remove_dir_recursively
is a non-trivial function that might use the pathname
buffers itself (this is _probably_ OK, as the likely culprit
would be calling resolve_gitlink_ref, but we do not pass the
proper flags to ask it to avoid blowing away gitlinks).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>