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

hg init
echo This is file a1 > a
hg add a
hg commit -m "commit #0" -d "1000000 0"
touch b
hg add b
rm b
hg commit -A -m"comment #1" -d "1000000 0"