Merge branch 'jk/notes-merge-from-anywhere'

Merge branch 'jk/notes-merge-from-anywhere'

"git notes merge" used to limit the source of the merged notes tree
to somewhere under refs/notes/ hierarchy, which was too limiting
when inventing a workflow to exchange notes with remote
repositories using remote-tracking notes trees (located in e.g.
refs/remote-notes/ or somesuch).

* jk/notes-merge-from-anywhere:
  notes: allow merging from arbitrary references