How to use filter-branch with --state-branch?

Recent versions of git filter-branch command introduced the --state-branch option. BTW I can't find any info about how this can be actually used.

We have this repository on github: https://github.com/concrete5/concrete5

When someone pushes to that repo, we clone it and execute `git filter-branch --subdirectory-filter concrete` to extract the concrete directory, and we push the result to https://github.com/concrete5/concrete5-core (including all the branches and tags)

The script at the moment is this one: https://github.com/concrete5/core_splitter/blob/70879e676b95160f7fc5d0ffc22b8f7420b0580b/bin/splitcore

I tried to use the --state-branch option on a local mirror, so that we could do an incremental filtering. Here's the script:

# Executed just one time
git clone --no-checkout --mirror \
   https://github.com/concrete5/concrete5.git work
cd work
git filter-branch \
   --subdirectory-filter concrete \
   --tag-name-filter cat \
   --prune-empty \
   --state-branch FILTERBRANCH_STATE \
   -- --all
# Executed every time the repo is updated
git remote update --prune
git filter-branch \
   --subdirectory-filter concrete \
   --tag-name-filter cat \
   --prune-empty \
   --state-branch FILTERBRANCH_STATE \
   -- --all

The first filter-branch call required 7168 steps, so did the second call... I also tried without the --prune option of remote update (I had to add --force to the second filter-branch), but nothing changed.