layouts: pull out logic for splitting svn paths into branch and local parts There is a single method on svnmeta that is responsible for both splitting a subversion path into a local component, which specifies a path relative to the mercurial root, and a branch component, which specifies the path to the root of a subversion branch, and translating that branch path into a mercurial branch name. This pulls the logic for doing the path splitting into a layout object method that *only* splits the path, and changes the svnmeta to call the layout objects's localname method to do the subversion branch path to mercurial branch name translation.