diff: report dirty submodules as changes in builtin_diff()
The diff machinery has two ways to detect changes to set the exit code: Just comparing hashes and comparing blob contents. The latter is needed if certain changes have to be ignored, e.g. with --ignore-space-change or --ignore-matching-lines. It's enabled by the diff_options flag diff_from_contents. The slower mode as never considered submodules (and subrepos) as changes with --submodule=diff or --submodule=log, which is inconsistent with --submodule=short (the default). Fix it. d7b97b7185 (diff: let external diffs report that changes are uninteresting, 2024-06-09) set diff_from_contents if external diff programs are allowed. This is the default e.g. for git diff, and so that change exposed the inconsistency much more widely. Reported-by: David Hull <david.hull@friendbuy.com> Signed-off-by: René Scharfe <l.s.r@web.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>