Ignore stderr with 'svn propget reviewboard:url .'

On my system the local en_US.UTF-8 does not exist (I have he_IL.UTF-8).
rbutils.utils.process.execute() forces the environment to be
LC_ALL=en_US.UTF-8, LANGUAGE=en_US.UTF-8. I can't override this from outside.
This means that svn will complain to the standad error about not being able to
set LC_CTYPE.

Furthermore, by default that function (execute()) will redirect stderr with
stdout. This resulted in the reviewbord server being '<long message from
svn><the URL>'.

This fix forces not redirecting the standard error there.

Reviewed at https://reviews.reviewboard.org/r/4670/