Deduplicate forks of forks

The way Git object deduplication works now (GitLab 11.9), we will only form deduplication pools around direct forks of an unforked project.

Example: suppose we have projects A and B. B is a fork of A. Now if a user creates a fork C of A, then C will join an object pool which includes at least A and C (possibly also B). However, if a user creates a clone D of B, then D will not join any object pool. D will just be an ordinary full copy of B. A fork of a fork won't get deduplicated.

We may want to change things so that forks of forks do get deduplicated.

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