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

hg init
touch a
hg add a
hg ci -m "a" -d "1000000 0"

echo 123 > b
hg add b
hg diff --nodates

hg diff --nodates -r tip

echo foo > a
hg diff --nodates