memctx: call super.__init__ instead of duplicating code

memctx: inherit from committablectx This patch marks the start of having memctx inherit from committablectx, thereby making it a full-fledged context that will eventually grow the ability to perform diffing and also merging.

committablectx: fix _manifest doc string

context: fix wrong indentation from renaming method

push: use bundle2 to push phases when available We now use a bundle2 container to push all phase updates at the same time. This is a significant step forward, even if further refactoring is needed to unify phase push with the changeset push.

pushkey: add an ``encode`` function This function is just a shorthand for ``encoding.fromlocal``. It will help hiding the encoding business from other code exchanging pushkey data over the wire.

bundle2: add ``pushkey`` support After ``listkeys`` we can now include ``pushkey`` request in a bundle2. The part uses a very simple scheme closest as possible to the current wireproto command for ``pushkey``. We may eventually decide for a more sophisticated part format before the protocol becomes final.

pushkey: add a ``decode`` function This function is just a shorthand for ``decoding.fromlocal``. It will help hiding the encoding business from other code exchanging pushkey data over the wire.

pull: when remote supports it, pull phase data alongside changesets We use bundle2 to retrieve the remote phase data at the same time as changesets. This reduces the amount of requestis and should improve consistency as the server can ensure nothing changed between the retrieval of those parts.

getbundle: support of listkeys argument when bundle2 is used A new ``listkeys`` is supported by getbundle. It is a list of namespaces whose content should be included in the bundle. An appropriate entry has been added to the wireproto map of getbundle arguments and a new bundle2 capability is advertised. There are still no codes that request such parts in core mercurial.

getbundle: raise error if extra arguments are provided for bundle10 New arguments are most likely to be supported by bundle20 (and above) only.

bundle: introduce a listkey handler This makes it possible to handle listkeys-related data stored in a bundle2. There are no producers yet.