subdirectory-filter does not delete files before the directory came into existence?

Hi,

There is a lot of information about extracting a directory from a git project. One thing I failed to find though is the following:

I try to extract a directory. The result is fine, but there is a lot of history in the result from *before* the directory was added to the project. Why? How can I get rid of this?

If you want to see yourself, I did:

	git clone git://www.swi-prolog.org/home/pl/git/pl-devel.git
	git clone pl-devel odbc
	cd odbc
	git filter-branch --subdirectory-filter packages/odbc --prune-empty
--tag-name-filter cat -- --all
	
Now use e.g. qgit to look at the history.  As from 03/07/2002, when
the packages/odbc directory was created, all looks just fine.  Before
though ...

	Thanks for any hints

		--- Jan