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
adding a
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
% push should push to default when default-push not set
pushing
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files
% push should push to default-push when set
pushing
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files