mpatch: write a cffi version of mpatch.patches

mpatch: remove dependency on Python.h in mpatch.c Now all the CPython-related stuff are referenced only from mpatch_module.c with mpatch.c being freely usable from a future cffi module

mpatch: split mpatch into two files

mpatch: provide things that will be exported later with a prefixed name For cffi a bunch of mpatch functions need to be visible through a .h file. This change renames them so it won't create potential c namespace conflicts.

mpatch: change Py_ssize_t to ssize_t in places that will be later copied

discovery: move code to create outgoing from roots and heads changegroup.changegroupsubset() contained somewhat low-level code for constructing an "outgoing" instance from a list of roots and heads nodes. It feels like discovery.py is a more appropriate location for this code. This code can definitely be optimized, as outgoing.missing will recompute the set of changesets we've already discovered from cl.between(). But code shouldn't be refactored during a move, so I've simply inserted a TODO calling attention to that.

bundle2: remove 'experimental.bundle2-exp' boolean config (BC) All users are migrated to 'devel.legacy.exchange', we can clean up the experimental namespace. Marking as (BC) because I know some large installation have bundle2 off and I want to make sure they notice the change.

tests: remove all remaining usage of experimental.bundle2-exp The only remaining usage of the experimental config were enforcing bundle2 on. These are very old remains of when bundle2 was off by default. This was also useful to highlight the fact that this was a bundle2 run and that a bundle1 one was nearby. However, we want a future developer working on bundle3 to notice possible output/behavior change on these tests and take them in account. So we do not enforce bundle2 for these runs. We leave a comment around to make sure dev still notice the bundle1 version.

tests: use 'legacy.exchange' option in various mixed tests The new option will stay around. The experimental option was only meant to be temporary. We update various tests that validate both bundle1 and bundle2 version side by side. This changeset only takes care of enforcing bundle1. The other use of 'experimental.bundle2-exp=True' will be taken care of in the next changeset.

tests: use 'legacy.exchange' option in various bundle1 tests The new option will stay around. The experimental option was only meant to be temporary.