histedit: move outgoing processing to its own function
Every piece of code extracted from the main command is a win. We simplify
changeset determination in the process. Parent ceases being a list before
becoming a node. We how have a root variable containing a node all the time.
To run the tests, do:
cd tests/
python run-tests.py
See http://mercurial.selenic.com/wiki/WritingTests for
more information on writing tests.