I don't end up with a 20GB gitolite admin repo

One command to run that:
- Stop system events (be carful with running event if any)
- Re-clone admin repo from remote ( git clone gitolite@gl-adm:gitolite-admin admin.new)
- Replace old admin repo by new (mv admin admin.old && mv admin.new admin)
- Set a flag in DB to know that we can run git gc
- Start system events

+ SYSTEM_CHECK that run git gc if the flag is set

=> new deployments must have git gc on by default (at plugin-git install ?)