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
hg -q
hg help
hg -q help
hg add -h
hg add --skjdfks
hg help diff
hg help status
hg -q help status
hg help foo
hg skjdfks

exit 0