convert: separate generic structures and variables from the filter specific ones

convert: separate generic structures and variables from the filter specific ones

To enable future reuse of the filter.<driver>.process infrastructure,
split the cmd2process structure into two separate parts.

subprocess_entry will now contain the generic data required to manage
the creation and tracking of the child process in a hashmap.

cmd2process is a filter protocol specific structure that is used to
track the negotiated capabilities of the filter.

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