overriding .subversion/config[helpers] on the command line

I have entries for diff3-cmd and merge-tool-cmd in the [helpers]
section of .subversion/config, and they seem to be working properly.
However, when running "svn merge --dry-run", I sometimes want all
merge and diff3 attempts to "fail" because I just want the report
of what files would be updated, without trying to actually merge
any files and especially without requiring any interaction.
I would prefer to specify this override on the command line rather
than having to edit, and subsequently restore, ~/.subversion/config.

It appears that I can override the diff3-cmd setting by adding
--diff3-cmd=/bin/false to the command, but when I try to do the
same thing to merge-tool-cmd (by adding --merge-tool-cmd=/bin/false)
I get an error message:

  svn: invalid option: --merge-tool-cmd=/bin/false

How do I go about overriding the merge-tool-cmd setting on the svn
command line? I found nothing pertinent in the svn 1.6 book, nor in
the archives.

svn version 1.6.17 (r1128011) on Ubuntu 12.04, in case it matters.

Please Cc.
Received on 2014-10-10 10:20:42 CEST