Following a blog, I created a batch file, wm.bat:
\"d:\\svnroot\\external\\winmerge\\WinMerge.exe\" /B /WAIT \"d:\\svnroot\\external\\winmerge\\WinMergeU.exe
After creating a batch file that contains a call to your favorite merge program, you can configure Subversion to always use your batch file in Windows (without requiring the --diff-cmd argument on each use) by modifying the line
# diff-cmd = diff_program (diff, gdiff, etc.)
in the file C:\Documents and Settings\username\Application Data\Subversion\config. This line should be changed to point to your batch file. For example:
diff-cmd = c:\bin\wm.bat