diff: make sure work tree side is shown as 0{40} when different

diff: make sure work tree side is shown as 0{40} when different

Ping Yin noticed that "git diff-index --raw" shows 0{40} when work tree
has submodule difference, but "git diff --raw" didn't correctly do so.

There was a mistake in the diffcore_skip_stat_unmatch() that was meant to
clean up the stat-only difference for running diff between the index and
work tree and diff between the tree and the work tree, to cause it re-read
from the submodule repository HEAD.  When ce_stat_match() says work tree
is different, we should always say 0{40} on the work tree side.

This patch fixes the issue, and adds tests.

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