diff: report copies and renames as changes in run_diff_cmd()

diff: report copies and renames as changes in run_diff_cmd()

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 has never considered copies and renames to be changes,
which is inconsistent with the quicker one.  Fix it.  Even if we ignore
the file contents (because it's empty or contains only ignored lines),
there's still the meta data change of adding or changing a filename, so
we need to report it in the exit code.

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: Jorge Luis Martinez Gomez <jol@jol.dev>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>