externally updated `_head` isn't detected in `ucm`

This can cause weirdness if there are two instances of ucm running backed by the same codebase.

I think the code to do anything about it is commented out (which we should fix) but there is code to display a warning, which also isn't firing.

If the external change leaves multiple heads, ucm should first:

• issue a message to the console, including the short hashes of the previous loopstate root hash, and of the newly detected heads (see reflog implementation for example of printing short hashes).
• merge the multiple heads (Branch.merge?)

Then (multiple heads or single head), ucm should:

• write the external change to the reflog
• update LoopState