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

changeset:   0:9426b370c206
tag:         tip
user:        My Name <myname@example.com>
date:        Mon Jan 12 13:46:40 1970 +0000
summary:     commit-1