Fix copy marking from diffcore-rename.

When (A,B) ==> (B,C) rename-copy was detected, we incorrectly said
that C was created by copying B.  This is because we only check if the
path of rename/copy source still exists in the resulting tree to see
if the file is renamed out of existence.  In this case, the new B is
created by copying or renaming A, so the original B is lost and we
should say C is a rename of B not a copy of B.

Signed-off-by: Junio C Hamano <junkio@cox.net>