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>