Xgit.Repository.WorkingTree.read_tree/3 could infinite loop on a malformed tree.

[scouten]

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Describe What Is Incomplete
The function Xgit.Repository.WorkingTree.read_tree/3 could go into an infinite loop if asked to read a malformed tree. The tree would need to point to itself or to one of its descendants. There is currently no protection against this.

Definition of Done
Construct a malformed tree (i.e. has a reference cycle). Call Xgit.Repository.WorkingTree.read_tree/3 with object_id set to that tree. A suitable error code should be returned rather than a timeout or stack-depth error.