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.
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
removing b
abort: update spans branches, use 'hg merge' or 'hg update -C' to lose changes