Enabling the gitaly_upload_pack_filter feature flag isn't enough for Partial clone

Using GitLab Enterprise Edition 12.8.0-ee from the omnibus packages on a Digital Ocean droplet, I found that what is documented on https://docs.gitlab.com/ee/topics/git/partial_clone.html doesn't actually work.

After enabling the feature flag in the rails console:

# gitlab-rails console
--------------------------------------------------------------------------------
 GitLab:       12.8.0-ee (0bd32f78864) EE
 GitLab Shell: 11.0.0
 PostgreSQL:   10.12
--------------------------------------------------------------------------------
Loading production environment (Rails 6.0.2)
irb(main):001:0> Feature.enable(:gitaly_upload_pack_filter)
=> nil`
irb(main):002:0> Feature.enabled?(:gitaly_upload_pack_filter)
=> true
irb(main):003:0>

I still get warning: filtering not recognized by server, ignoring when cloning with git clone --filter='blob:limit=1m' --no-checkout git@64.227.74.213.nip.io:christian/git.git, and the files that should be excluded by the filter are actually part of the clone.

If I manually change the config on the server using:

/opt/gitlab/embedded/bin# ./git config --system uploadpack.allowFilter true
/opt/gitlab/embedded/bin# ./git config --system uploadpack.allowAnySHA1InWant true

it actually works correctly. I don't get warning: filtering not recognized by server, ignoring and the files that should be excluded are actually excluded.

cc @jramsay