Rewriting history with git-filter-branch and leaking objects (?)

Hi,

I've been playing with git-filter-branch, and was wondering how objects
from the original branch are supposed to be removed.

It looks like removing the refs/original/* refs is not enough.

And it also looks like when all references seem to be removed, git-prune
doesn't fully do its job...

See the following transcript:

$ git init
Initialized empty Git repository in .git/
$ echo a > a ; echo b > b
$ git add a b
$ git commit -m "add a b"
Created initial commit b8875b1: add a b
 2 files changed, 2 insertions(+), 0 deletions(-)
 create mode 100644 a
 create mode 100644 b
$ echo a >> a
$ git commit -a -m "update a"
Created commit fd97ed9: update a
 1 files changed, 1 insertions(+), 0 deletions(-)
$ git-filter-branch --index-filter 'git-update-index --remove b' HEAD
Rewrite fd97ed9a2fef62eca824361fb62269e3c1fc0fb8 (2/2)
Ref 'refs/heads/master' was rewritten

These refs were rewritten:
fatal: Not a git repository: '/tmp/test/.git-rewrite/t/../../.git'

(This is a message that happens when GIT_DIR is not set, I guess the
patches that were sent a few days ago on the list fix this issue)

$ git-cat-file commit b8875b1
tree 3683f870be446c7cc05ffaef9fa06415276e1828
author Mike Hommey [off-list ref] 1187369087 +0200
committer Mike Hommey [off-list ref] 1187369087 +0200

add a b

(not a surprise, since we still have the refs/original/refs/heads/master
ref)

$ git-update-ref -d refs/original/refs/heads/master fd97ed9
$ git-cat-file commit b8875b1
tree 3683f870be446c7cc05ffaef9fa06415276e1828
author Mike Hommey [off-list ref] 1187369087 +0200
committer Mike Hommey [off-list ref] 1187369087 +0200

add a b
$ git fsck

(okay, so it is still here, and obviously still referenced ; it appears
to be referenced in .git/logs/...)

$ rm .git/logs/refs/heads/master 
$ rm .git/logs/HEAD
$ git fsck
dangling commit fd97ed9a2fef62eca824361fb62269e3c1fc0fb8

(finally ! So here is a first question: is there a proper way to clean
this out ? rm of the logs sounds brutal...)

$ git-prune -n
3683f870be446c7cc05ffaef9fa06415276e1828 tree
b8875b1095616c1e7e8f8ffce8ebc172059367ea commit
fd97ed9a2fef62eca824361fb62269e3c1fc0fb8 commit
$ git-cat-file commit fd97ed9a2fef62eca824361fb62269e3c1fc0fb8
tree c1f89248e4b6e47a4529d50d37b0840a14d2efb0
parent b8875b1095616c1e7e8f8ffce8ebc172059367ea
author Mike Hommey [off-list ref] 1187369110 +0200
committer Mike Hommey [off-list ref] 1187369110 +0200

update a

(Why doesn't prune -n tell me it would remove
c1f89248e4b6e47a4529d50d37b0840a14d2efb0, which it should, AFAIK ?)

Mike