@ECHO off

SETLOCAL enabledelayedexpansion

SET svnFolders=

FOR /D %%G IN ("C:\Users\xyz\Documents\abcd\*") DO (
    IF EXIST %%G"\.svn" (
        SET svnFolders=!svnFolders!%%G*
    )
)

REM echo %svnFolders%
START TortoiseProc.exe /command:update /path:%svnFolders% /closeonend:0