Fix for *svn-status* buffer issue

Hi,

Powerline didn't work in **\*svn-status*** buffer (`M-x svn-status` command of `psvn.el`). The modeline went blank and the **\*Messages*** buffer had a new message:

    Error during redisplay: (wrong-type-argument sequencep svn-status-mode-line-process) [X times]

It appears that `psvn.el` sets the value of `mode-line-process` variable like this:

      (setq mode-line-process 'svn-status-mode-line-process)

This commit seems to fix the issue. However, my elisp knowledge is limited, so if there is a better way to handle this, please do so.

Thanks,
Prakash