rust-tuf doesn't support diamond delegations

[erickt]

While writing a delegation builder, I noticed that while we have a test for diamond delegations, it doesn't actually work. Consider this delegation tree:

 targets
 /     \
A       B
 \     /
    D

Where D is signed correctly by B, but not signed correctly by A.

Both Client::lookup_target_description and Client::target_description_with_start_time will visit:

• targets
• A
• D (error, A did not sign D. marking as visited)
• B
• skip D since it was already visited.

We need to update these delegations as visited only if we decided the delegation is trusted.