repository: formalize interfaces for revision deltas and requests Now that we've sufficiently abstracted how revision deltas are produced in changegroup code, we can now start the process of formalizing that as part of the interfaces defined in the repository module. This commit essentially converts the revisiondelta and revisiondeltarequest classes into well-defined interfaces. This is not strictly necessary. But I want all types formalized by the storage interface to have interfaces. This makes it much easier to test for interface conformance and for implementing new storage backends. Because the interface is documented, comments and docstrings from changegroup.py have been dropped. Differential Revision: https://phab.mercurial-scm.org/D4225