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