git filter-branch --directory-filter oddity

When using git filter-branch --prune-empty --directory-filter foo/bar to extract the history of the foo/bar directory, I am getting a very strange result.

Directory foo/bar is "slow moving". Say, 22 commits out of several thousand. I would like to extract just those 22 commits.

Instead, I get ~1500 commits, which seem to have not been skipped because they are merge commits. Merges completely immaterial to this directory.

As they have not been skipped, they are fully fleshed out. By this, I mean that we have the whole tree in place. So these 22 commits appear with foo/bar pulled out to the root of the project, in the midst of 1500 commits with a full tree.

The commit diffs make no sense what-so-ever.

Am I doing it wrong?

m