restructure changelog file appending - make appending code proper part of changelog with delayupdate/finalize - use simplified appender that tracks pending data in memory - eliminate old appendfile and helper classes - update addchangegroup to use new interface and reuse the existing changelog

To run the tests, do:

cd tests/
python run-tests.py

See http://www.selenic.com/mercurial/wiki/index.cgi/WritingTests for
more information on writing tests.