docs: def generate_control_field(self, revision=None):
          """
          Generate a Debian control file field referring for this repository and revision.
  
          :param revision: A reference to a revision, most likely the name of a
                           branch (a string, defaults to :attr:`default_revision`).
          :returns: A tuple with two strings: The name of the field and the value.
  
          This generates a `Vcs-Bzr` field for Bazaar repositories, a `Vcs-Git`
          field for Git repositories and a `Vcs-Hg` field for Mercurial
          repositories. Here's an example based on the public git repository of
          the `vcs-repo-mgr` project:
  
          >>> from vcs_repo_mgr import coerce_repository
          >>> repository = coerce_repository('https://github.com/xolox/python-vcs-repo-mgr.git')
          >>> repository.generate_control_field()
          ('Vcs-Git', 'https://github.com/xolox/python-vcs-repo-mgr.git#b617731b6c0ca746665f597d2f24b8814b137ebc')
          """
          value = "%s#%s" % (self.remote or self.local, self.find_revision_id(revision))
          return self.control_field, value