convert: workaround for cvsps.cache under Windows In non-cygwin environment, cvsps fails to create its cache directory and redirect its output to stderr. Just ignore the error and capture stderr as well.

adding b
   rev    offset  length   base linkrev nodeid       p1           p2
     0         0       3      0       0 1e88685f5dde 000000000000 000000000000