subrepo/svn: make rev number retrieval compatible with svn 1.5 (issue2968) ae2664ee0223 introduced "svn info TARGET@REV" to determine if a certain path exists in the specified revision, but in svn 1.5 the error message "Not a valid URL" yields exit code 0 so the error is not caught. Use "svn list TARGET@REV" instead which works with svn 1.5 and is even faster in some situations.