fix traceback of extdiff after a merge - files that comes from a different branch are marked as modified but aren't present in the original manifest - add a testcase for extdiff and for regular diff

#!/bin/sh

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

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

hg -q help help