pack-refs: teach --exclude option to exclude refs from being packed

pack-refs: teach --exclude option to exclude refs from being packed

At GitLab, we have a system that creates ephemeral internal refs that
don't live long before getting deleted. Having an option to exclude
certain refs from a packed-refs file allows these internal references to
be deleted much more efficiently.

Add an --exclude option to the pack-refs builtin, and use the ref
exclusions API to exclude certain refs from being packed into the final
packed-refs file

Signed-off-by: John Cai <johncai86@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>