git --diff: fix traceback when getting mode change - use the manifest instead of the mode in the working dir if the diff is against two revisions - add a testcase

#!/bin/sh

echo 'syntax error' > badext.py
abspath=`pwd`/badext.py

echo '[extensions]' >> $HGRCPATH
echo "badext = $abspath" >> $HGRCPATH

hg -q help help