nmbug: Handle missing @upstream in is_unmerged

If we don't have an upstream, there is nothing to merge, so nothing is
unmerged.  This avoids errors like:

  $ nmbug status
  error: No upstream configured for branch 'master'
  error: No upstream configured for branch 'master'
  fatal: ambiguous argument '@{upstream}': unknown revision or path not in the working tree.
  Use '--' to separate paths from revisions, like this:
  'git <command> [<revision>...] -- [<file>...]'
  'git rev-parse @{upstream}' exited with nonzero value

You might not have an upstream if you're only using nmbug locally to
version-control your tags.