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

mkdir t
cd t
hg init
echo a > a
hg add a
hg commit -m test -d "1000000 0"
hg history
hg manifest
hg cat a
hg verify