py3: fix sorting of obsolete markers when building bundle
Last item of marker tuple (parents) is either None or tuple. Comparison
thus fails on Python 3 with:
TypeError: '<' not supported between instances of 'tuple' and 'NoneType'
Fixing this by coercing None to the empty tuple when sorting markers in
exchange._getbundleobsmarkerpart().
#require emacs
$ emacs -q -no-site-file -batch -l $TESTDIR/../contrib/hg-test-mode.el \
> -f ert-run-tests-batch-and-exit
Running 1 tests (*) (glob)
passed 1/1 hg-test-mode--compilation-mode-support
Ran 1 tests, 1 results as expected (*) (glob)