dirstate._dirs: fix refcounting broken by 7dfac37cfabf
reported by Patrick Waugh
# should print nothing# should failabort: path 'b/x' is inside repo 'b'abort: path 'b/x' is inside repo 'b'# should failabort: path 'b/x' is inside repo 'b'# should arguably print nothing# should failabort: path 'b/a' is inside repo 'b'