run-command: wean start_command() off the_repository

run-command: wean start_command() off the_repository

The start_command() relies on the_repository due to the
close_object_store flag in 'struct child_process'. When this flag is
set, start_command() closes the object store associated with
the_repository before spawning a child process.

To eliminate this dependency, replace the 'close_object_store' with the
new 'struct object_database *odb_to_close' field. This allows callers to
specify the object store that needs to be closed.

Suggested-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Burak Kaan Karaçay <bkkaracay@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>