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__)