def find_revision_number(self, revision=None):
raise NotImplementedError()<EOL>
Find the local revision number of the given revision.

:param revision: A reference to a revision, most likely the name of a
                 branch (a string, defaults to :attr:`default_revision`).
:returns: The local revision number (an integer).

This method needs to be implemented by subclasses:

- With each commit that is added to the repository, the local revision
  number needs to increase.

- Whether revision numbers start counting from zero or one is left to
  the version control system. To make things more concrete: While
  Bazaar and git count from one, Mercurial counts from zero.