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
1 files updated, 0 files merged, 0 files removed, 0 files unresolvedremoving babort: update spans branches, use 'hg merge' or 'hg update -C' to lose changes