Fetch notes

[znz]

Fix error on export_changelog:

% docker run -it -v $(pwd)/pkg:/root/pkg ruby-snapshot bundle exec rake snapshot
Already on 'master'
From https://github.com/ruby/ruby
 * branch                  master     -> FETCH_HEAD
Cloning into '/tmp/ruby-snapshot20190826-37-je8moq/ruby'...
done.
Checking out files: 100% (9355/9355), done.
Traceback (most recent call last):
	6: from ruby/tool/make-snapshot:523:in `<main>'
	5: from ruby/tool/make-snapshot:523:in `collect'
	4: from ruby/tool/make-snapshot:523:in `block in <main>'
	3: from ruby/tool/make-snapshot:351:in `package'
	2: from ruby/tool/make-snapshot:351:in `chdir'
	1: from ruby/tool/make-snapshot:353:in `block in package'
/root/ruby/tool/lib/vcs.rb:573:in `export_changelog': need notes/commits tree; run `git fetch origin refs/notes/commits:refs/notes/commits` in the repository (RuntimeError)
rake aborted!