Some "git notes" operations, e.g. "git log --notes=<note>", should
be able to read notes from any tree-ish that is shaped like a notes
tree, but the notes infrastructure required that the argument must
be a ref under refs/notes/. Loosen it to require a valid ref only
when the operation would update the notes (in which case we must
have a place to store the updated notes tree, iow, a ref).
* mh/notes-allow-reading-treeish:
notes: allow treeish expressions as notes ref