Fork storage utilization and reporting
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### Problem to solve
[Git object deduplication for forks](https://docs.gitlab.com/ee/development/git_object_deduplication.html) reduces storage costs and can improve forking speeds.
How deduplicated repositories report disk utilization and how this applies to storage limits is not well defined. This creates problems like https://gitlab.com/gitlab-org/gitlab/-/issues/255001
### Further details
### Proposal
### Links / references
issue