diff: drop dry-run redirection to /dev/null
As an added protection against dry-run diffs accidentally producing output, we redirect diff_options.file to /dev/null. But as of the previous patch, this now does nothing, since dry-run diffs are implemented by setting "file" to NULL. So we can drop this extra code with no change in behavior. This is effectively a revert of 623f7af284 (diff: restore redirection to /dev/null for diff_from_contents, 2025-10-17) and 3da4413dbc (diff: make sure the other caller of diff_flush_patch_quietly() is silent, 2025-10-22), but: 1. We get a conflict because we already dropped the color_moved handling in an earlier patch. But we just resolve the conflicts to "theirs" (removing all of the code). 2. We retain the test from 623f7af284. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>