obshistory: only display each revision once in debugobshistory
When using a revision range, the same precursor could have been
displayed more than once. Use a cache to display each revision
only once.
It's a fix until we found the nicest way to display obs history.
# name space package to host third party extensions
from __future__ import absolute_import
import pkgutil
__path__ = pkgutil.extend_path(__path__, __name__)