Re: [PATCH 3/4] diff: replace diff_options.dry_run flag with NULL file

Jeff King [off-list ref] writes:

We introduced a dry_run flag to diff_options in b55e6d36eb (diff: ensure
consistent diff behavior with ignore options, 2025-08-08), with the idea
that the lower-level diff code could skip output when it is set.

As we saw with the bugs fixed by 3ed5d8bd73 (diff: stop output garbled
message in dry run mode, 2025-10-20), it is easy to miss spots. In the
end, we located all of them by checking where diff_options.file is used.

That suggests another possible approach: we can replace the dry_run
boolean with a NULL pointer for "file", as we know that using "file" in
dry_run mode would always be an error. This turns any missed spots from
producing extra output[1] into a segfault. Which is less forgiving, but
that is the point: this is indicative of a programming error, and
complaining loudly and immediately is good.

Makes sense.