Transplant branch from one repo to another

Hello all,

I have a repository cloned from SVN using 'git svn clone http://servername/path
-T trunk -b branches -t tags', or thereabouts, and I've since realised that I
would have been better off with 'git svn clone
http://servername/path/trunk/subdir', as I don't really care about any other
branches, or about the contents of the svn repository outside of 'subdir'.

I have one branch from trunk in which I've been working, which includes one
merge from trunk. I'm interested in how I might be able to get the content of
that branch and transplant it onto the new svn clone, starting at the equivalent
parent commit (ie. the same svn revision). The merge was only a few commits away
from HEAD, so I can manually redo that last part if it complicates things too
much.

I've been thinking about something involving 'filter-branch
--subdirectory-filter' followed by a fast-export on the source, and then a
fast-import on the destination, but I'm not sure I'm going about this the right
way and would appreciate any advice anyone can give me.

So, does anybody have any suggestions for how I should do this?
Thank you for your time,
Nye