format-patch: refactor output selection

The --stdout and --output-directory options are mutually exclusive, but
it's hard to tell from reading the code. We have three separate
conditionals that check for use_stdout, and it's only after we've set up
the output_directory fully that we check whether the user also specified
--stdout.

Instead, let's check the exclusion explicitly first, then have a single
conditional that handles stdout versus an output directory. This is
slightly easier to follow now, and also will keep things sane when we
add another output mode in a future patch.

We'll add a few tests as well, covering the mutual exclusion and the
fact that we are not confused by a configured output directory.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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