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
0 files updated, 0 files merged, 0 files removed, 0 files unresolved
0 files updated, 0 files merged, 0 files removed, 0 files unresolved
adding b
0 files updated, 0 files merged, 0 files removed, 0 files unresolved
adding b