Since write_ref_sha1() can only write loose refs and cannot write
symbolic refs, there is no need for it to invalidate the packed ref
cache.
Suggested by: Martin Fick <mfick@codeaurora.org> Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu> Signed-off-by: Junio C Hamano <gitster@pobox.com>