format-patch: fix bug with --stdout in a subdirectory

format-patch: fix bug with --stdout in a subdirectory

We set the output directory to the git subdirectory prefix if one has
not already been specified. However, in the case of --stdout, we
explicitly _don't_ want the output directory to be set. The result was
that "git-format-patch --stdout" in a directory besides the project root
produced the "standard output, or directory, which one?" error message.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>