obslog: add header to the changeset description diff This patch adds `diff -r <hash> -r <hash> <filename>` header to the output of the changeset description diff which is shown using the --patch flag of obslog command. This fixes the label/color issue for description diff also.

# name space package to host third party extensions
from __future__ import absolute_import
import pkgutil
__path__ = pkgutil.extend_path(__path__, __name__)