Fix vc-svn.el 1.27 when using with GNU Emacs 21.4

[Henning Schmiedehausen]

Please apply the attached patch. Its function is obvious. As
vc-do-command only returns 'nil' or 'error', the status is never 0. As a
result, vc-svn.el has been broken for a while when using with emacs
21.4.

        Best regards
                Henning


-- 
Dipl.-Inf. (Univ.) Henning P. Schmiedehausen          INTERMETA GmbH
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