Gitaly needs to partition pools, source repositories and forks in the
same partition. Generally the RPCs creating the repositories or the
relationships carry an additional repository that is depending on the
RPC either the pool, fork, or the source repository. This enables
Gitaly to figure out which partition the target repository should be
placed. In some cases though, the RPC doesn't contain this context.
For example, ReplicateRepository first replicates the object pool and
then the linked repository. The ReplicateRepository request itself
doesn't carry enough information to determine which partition a given
repository should go into but the caller knows whether the repository
being replicated will be subsequently linked to a pool. To account
for such scenarios, this commit starts considering partitioning hints
sent by the client when creating partitiong assignments. The client
sends a relative path that the target repository should be placed in
the same partition with. Sending such a hint will then lead to both
repositories being in the same partition and enable linking them
together later.