Windows: start_command: Support non-NULL dir in struct child_process

Windows: start_command: Support non-NULL dir in struct child_process

A caller of start_command can set the member 'dir' to a directory to
request that the child process starts with that directory as CWD. The first
user of this feature was added recently in eee49b6 (Teach diff --submodule
and status to handle .git files in submodules).

On Windows, we have been lazy and had not implemented support for this
feature, yet. This fixes the shortcoming.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>