changectx: if passing a basectx then exit __init__ immediately

basectx: return a copied context if changeid is already a basectx This implements a copy constructor so that we can pass a basectx-derived object in future refactorings.

basectx: add an empty class that will be used as a parent of all contexts At the moment, there is no simple way to check if an object is a context because there is no common parent class. If there were, we could use 'isinstance' everywhere. Simply having memctx inherit from workingctx or changectx would allow the use of 'isinstance' but that could lead to some confusing situations of reading the code since we have three distinct concepts of a context: - changectx represents a changeset *already* in the repo, and is therefore immutable - workingctx represents changes on disk in the working directory - memctx represents changes solely in memory which may or may not be on disk Therefore, I propose refactoring context.py to have all three contexts inherit from a parent class 'basectx'.