Revert your local svn checkout, then make changes to your local copy.
Then svn diff
from trunk and there you go...
svn diff tools/whatever.vbs
will tell svn to diff just that file.
the redirectional operator as always (>) redirects stdout to a plain
text file, which is your patch.

-- 
Daniel Dalton
http://members.iinet.net.au/~ddalton/
<d.dalton_at_iinet.net.au>