fix a bug in the rename code in undo.c: do assume the sfile exist, it may be removed by stripdel.
fix a bug in the rename code in undo.c:
do assume the sfile exist, it may be removed by stripdel.

bk: 38c0b264cWDanQa2qICe5Hg4YHFJKQ
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