subrepo: handle svn tracked/unknown directory collisions This happens more often than expected. Say you have an svn subrepository with python code. Python would have generated unknown .pyc files. Now, you rebase this setup on a revision where a directory containing python code does not exist. Subversion is first asked to remove this directory when updating, but will not because it contains untracked items. Then it will have to bring back the directory after the merge but will fail because it now collides with an untracked directory. Using --force is not very elegant but it is much simpler than rewriting our own purge command for subversion.