transport: free filter options in disconnect_git()

transport: free filter options in disconnect_git()

If a user of the transport API calls transport_set_option() with
TRANS_OPT_LIST_OBJECTS_FILTER, it doesn't pass a struct, but rather a
string with the filter-spec, which the transport code then stores in its
own list_objects_filter_options struct.

When the caller is done and we call transport_disconnect(), the contents
of that filter struct are then leaked. We should release it before
freeing the transport struct.

Another way to solve this would be for transport_set_option() to pass a
pointer to the struct. But that's awkward, because there's a generic
transport-option interface that always takes a string. Plus it opens up
questions of memory lifetimes; by storing its own filter-options struct,
the transport code remains self-contained.

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