fix a fd leak in bk undo  (cuased problem in win32 with :bk -sa<top-of-trunk>)
fix a fd leak in bk undo  (cuased problem in win32 with :bk -sa<top-of-trunk>)

bk: 3b8acf0bGZXTKcrJSDD1SS7LLYV4_g
bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper