convert: split start_multi_file_filter() into two separate functions

convert: split start_multi_file_filter() into two separate functions

To enable future reuse of the filter.<driver>.process infrastructure,
split start_multi_file_filter() into two separate parts.

start_multi_file_filter() will now only contain the generic logic to
manage the creation and tracking of the child process in a hashmap.

start_multi_file_filter_fn() is a protocol specific initialization
function that will negotiate the multi-file-filter interface version
and capabilities.

Signed-off-by: Ben Peart <benpeart@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>