mingw: demonstrate that all file handles are inherited by child processes

mingw: demonstrate that all file handles are inherited by child processes

When spawning child processes, we really should be careful which file
handles we let them inherit.

This is doubly important on Windows, where we cannot rename, delete, or
modify files if there is still a file handle open.

Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>