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
% file replaced with directory
adding a
% should fail - would corrupt dirstate
abort: file named 'a' already in dirstate
% directory replaced with file
adding a/a
% should fail - would corrupt dirstate
abort: directory named 'a' already in dirstate
% directory replaced with file
adding b/c/d
% should fail - would corrupt dirstate
abort: directory named 'b' already in dirstate