In walk(), don't die when os.lstat() raises os.error, e.g. because a

In walk(), don't die when os.lstat() raises os.error, e.g. because a
file was deleted by a previous call to the visitor function.

This used to be the behavior in 1.5.2 and before, but a patch to avoid
making two stat() calls accidentally broke this in 2.0.

Moshe, this would be a good one for 2.0.1 too!