sub-process: separate process lifecycle from hashmap management

sub-process: separate process lifecycle from hashmap management

subprocess_start() and subprocess_stop() couple two concerns:
managing a child process (setup, handshake, teardown) and
managing a hashmap that indexes running processes by command
string.  The hashmap suits callers like convert.c where many
files may share one filter process looked up by name, but
callers that manage process lifetime through their own data
structures do not need it.

Extract subprocess_start_command() and subprocess_stop_command()
so callers can reuse the child process setup and handshake
machinery without maintaining a hashmap.  subprocess_start()
and subprocess_stop() become thin wrappers that add hashmap
operations on top.

Signed-off-by: Michael Montalbo <mmontalbo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>