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
echo This is file a1 > a
hg add a
hg commit -m "commit #0" -d "1000000 0"
touch b
hg add b
rm b
hg commit -A -m"comment #1" -d "1000000 0"