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
mkfifo p

hg serve --stdio < p &
P=$!
(echo lock; echo addchangegroup; sleep 5) > p &
Q=$!
sleep 3
kill -HUP $P
wait
ls .hg