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

mkdir t
cd t
hg init
echo a > a
hg add a
hg commit -m "test" -d "1000000 0"
hg verify
hg parents
hg status
hg rollback
hg verify
hg parents
hg status