diff: replace diff_options.dry_run flag with NULL file

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.

[1] We protect ourselves against garbled output as a separate step,
    courtesy of 623f7af284 (diff: restore redirection to /dev/null for
    diff_from_contents, 2025-10-17). So in that sense this patch can
    only introduce user-visible errors (since any "bugs" were going to
    /dev/null before), but the idea is to catch them rather than quietly
    send garbage to /dev/null.

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

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