bundle2: track life cycle of parts We introduce a ``_generated`` attribute on parts. Coming changesets will make it easier to update a part's contents after its creation. We need a way to track if the part is still open to modification or if it is currently being generated and should not be touched anymore. As a bonus, we can now detect and crash if someone manages to write bogus code to get a part generated twice.

bundle2: update all ``addpart`` callers to ``newpart`` The new method is what we want in all current cases.

bundle2: have ``newpart`` automatically add the part to the bundle The created part is automatically added to the bundle as this is most certainly the intent of the user code.

bundle2: add a ``newpart`` method to ``bundle20`` Creating new parts is the most common operation people do when exposed to a bundler. We create a dedicated method on the bundler object for it. This will simplify the code and also avoid having to import the ``mercurial.bundle2`` module in multiple places. One part creators have been updated for testing purpose.

bundle2: small doc update on the bundler The `bundle20` class contains methods to help define the content and methods to generate the actual stream. We add small doc headers to help distinguish between the two.

localrepo: replace status method with a shim The path of the righteous man is beset on all sides by the inequities of the selfish and the tyranny of evil men. Blessed is he, who in the name of charity and good will, shepherds the weak through the valley of darkness, for he is truly Mercurial's keeper and the finder of robust methods. And I will strike down upon thee with great vengeance and furious anger those who would attempt to poison and destroy Mercurial's codebase. And you will know my name is the Lord when I lay my vengeance upon thee.

workingctx: have status method call super instead of customized code. The old code is unneeded now that basectx has the ability to calculate the status between two context objects. Now, we use the objected oriented pattern called 'super'.

basectx: copy localrepo.status method Now that all the pieces are in place, we copy the status method from localrepo. In the next few patches, we will remove the old implementation.

committablectx: cache _status in _poststatus A future patch will remove the old workingctx.status which caches the status of the working directory, therefore we now cache this status in the poststatus hook of committablectx.

committablectx: simplify caching the status Previously, workingctx had custom variables for the unknown, ignored, and clean list of files of status. These then got moved to committablectx and, after the refactoring of localrepo.status, are no longer needed. We, therefore, simplify the whole mess. As a bonus, we are able to remove the need for having 'assert'.