format-patch: support --output option

We've never intended to support diff's --output option in format-patch. And until baa4adc66a (parse-options: disable option abbreviation with PARSE_OPT_KEEP_UNKNOWN, 2019-01-27), it was impossible to trigger. We first parse the format-patch options before handing the remainder off to setup_revisions(). Before that commit, we'd accept "--output=foo" as an abbreviation for "--output-directory=foo". But afterwards, we don't check abbreviations, and --output gets passed to the diff code. This results in nonsense behavior and bugs. The diff code will have opened a filehandle at rev.diffopt.file, but we'll overwrite that with our own handles that we open for each individual patch file. So the --output file will always just be empty. But worse, the diff code also sets rev.diffopt.close_file, so log_tree_commit() will close the filehandle itself. And then the main loop in cmd_format_patch() will try to close it again, resulting in a double-free. The simplest solution would be to just disallow --output with format-patch, as nobody ever intended it to work. However, we have accidentally documented it (because format-patch includes diff-options). And it does work with "git log", which writes the whole output to the specified file. It's easy enough to make that work for format-patch, too: it's really the same as --stdout, but pointed at a specific file. We can detect the use of the --output option by the "close_file" flag (note that we can't use rev.diffopt.file, since the diff setup will otherwise set it to stdout). So we just need to unset that flag, but don't have to do anything else. Our situation is otherwise exactly like --stdout (note that we don't fclose() the file, but nor does the stdout case; exiting the program takes care of that for us). Reported-by: Johannes Postler <johannes.postler@txture.io> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>