diff: report dirty submodules as changes in builtin_diff()

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>