treemanifest: extract parse method from constructor
When we start to lazily load submanifests, it will be useful to be
able to create an treemanifest instance before manifest data gets
parsed into it. To prepare for this, extract the parsing code from
treemanifest's constructor to a separate method.
To run the tests, do:
cd tests/
python run-tests.py
See http://mercurial.selenic.com/wiki/WritingTests for
more information on writing tests.