One possible way to determine how much of the UUID we need to keep
for uniqueness:

* Read the issue database into memory

* Build a tree of all the issue ids:

    * Define a Node type for a tree datastructure as:
        enum Node {
            Empty,
            IssueId(String),
            SubNodes([&Node; 256]),
        }

    * Create an empty root node = Node::Empty

    * For each issue:
        issue_id_bytes = issue_id.iter()

        issue_id_byte = issue_id_bytes.next()
        current_node = root

        while current_node == SubNodes:
            current_node = current_node[issue_id_byte]
            issue_id_byte = issue_id_bytes.next()

        if current_node == Empty:
            current_node = Node::IssueId(issue_id)
        else: # we know current_node == IssueId(original_issue_id)
            replace current_node with a SubNodes initialized to [Node::Empty; 256]
            current_node[original_issue_id[depth]] = Node::IssueId(original_issue_id)
            recurse trying again to insert issue_id

* Walk the entire tree, keeping track of the depth of each IssueId node (ie the number of SubNodes nodes above it)

* The largest depth is the number of bytes of the issue ids neede to ensure uniqueness