clone: make filter_options local to cmd_clone()

clone: make filter_options local to cmd_clone()

The `struct list_objects_filter_options filter_options` variable used
in "builtin/clone.c" to store the parsed filters specified by
`--filter=<filterspec>` is currently a static variable global to the
file.

As we are going to use it more in a following commit, it could become
a bit less easy to understand how it's managed.

To avoid that, let's make it clear that it's owned by cmd_clone() by
moving its definition into that function and making it non-static.

The only additional change to make this work is to pass it as an
argument to checkout(). So it's a small quite cheap cleanup anyway.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>