Another user (Sergey Azarkevich) actually pointed me to an interesting fact:
C:\test\test2checkout>svn merge file:///C:/test/test2/A B --record-only
--- Recording mergeinfo for merge of r2 through r5 into 'B':
...
C:\test\test2checkout>svn merge file:///C:/test/test2/A B
--- Merging r3 through r5
Looks like the batch-file got truncated by some clients/mail servers on
the way --- here's the plain batch file content.
Anyone having an idea what's going on here?
REM create test repository
mkdir C:\test
cd /d C:\test
mkdir test2
svnadmin create test2
REM check-out test repository
mkdir test2