mailinfo: explicitly close file handle to the patch output

mailinfo: explicitly close file handle to the patch output

This does not make a difference within the context of "git mailinfo"
that runs once and exits, as flushing and closing would happen upon
process termination.  It however will matter when we eventually make
it callable as an API function.

Besides, cleaning after yourself once you are done is a good hygiene.

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