diff: teach "--stat -w --exit-code" to notice differences

diff: teach "--stat -w --exit-code" to notice differences

When options like "-w" is used while "--exit-code" option is in
effect, instead of the usual "do we have any filepair whose preimage
and postimage have different <mode,object>?" check, we need to compare
the contents of the blobs, taking into account that certain changes
are considered no-op.

With the previous step, we taught "--patch" codepath to set the
.found_changes bit correctly, even for a change that only affects
the mode and not object.  The "--stat" codepath, however, did not
set the .found_changes bit at all.  This lead to

    $ git diff --stat -w --exit-code

for a change that does have an output to exit with status 0.

Set the bit by inspecting the list of paths the diffstat output is
given for (a mode-only change will still appear as a "0-line added
0-line deleted" change) to fix it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>