subdir breaks `librarian-puppet update`

[haf]

Repro:

1. create a new Puppetfile, with a git ref with a subdir
2. run librarian-puppet update

error: pathspec 'origin/master' did not match any file(s) known to git.
/Users/henfel/.rvm/gems/ruby-1.9.3-p374@logibit-receipts/gems/librarian-puppet-simple.haf-0.1.1/lib/librarian/puppet/simple/util.rb:40:in `system_cmd': Cmd git fetch origin && git checkout origin/master failed (StandardError)

[haf]

This is with the sent PR that adds support for subdirs.