rust-discovery: implementing and exposing stats() This time, it's simple enough that we can do it in all layers in one shot. Differential Revision: https://phab.mercurial-scm.org/D6233

rust-discovery: cpython bindings for the core logic As previously done with the ancestors submodule, testing for the bindings is provided from Python on a trivial case. Differential Revision: https://phab.mercurial-scm.org/D6232

rust-discovery: starting core implementation Once exposed to the Python side, this core object will avoid costly roundtrips with potentially big sets of revisions. This changeset implements the core logic of the object only, i.e., manipulation of the missing, common and undefined set-like revision attributes. Differential Revision: https://phab.mercurial-scm.org/D6231

rust-dagops: roots Unsuprisingly, the algorithm is much easier than for heads, provided we work on a set in the first place. To improve the signature, a trait for set-likes object would be useful, but that's not an immediate concern. Differential Revision: https://phab.mercurial-scm.org/D6230

rust-dagops: range of revisions This is a Rust implementation for what reachableroots2() does if includepath is True. The algorithmic details and performance notes are included in the documentation comment. Our main use case for now is a Rust counterpart of the partialdiscovery object, so we don't really need bindings yet. Differential Revision: https://phab.mercurial-scm.org/D6229

setup: tweak error message for Python 3 We now have beta support for Python 3. In my opinion, it isn't yet stable enough to allow `pip install Mercurial` to work with Python 3 out of the box: we don't want people accidentally using Mercurial with Python 3 just yet. But I do think we should be more friendly about informing people of their options. This commit tweaks the error message that users see when running setup.py with Python 3. We instruct them about the current level of Python 3 support, point them at the wiki for more info, and give them instructions on how to bypass the check. As part of this, I also changed which version value is printed, as we were printing a named tuple before.

setup: remove set and dict comprehensions Yuya observed in a recent review that it is worthwhile to keep setup.py parseable with Python 2.6 so a useful error message is seen when attempting to run with Python 2.6. This commit removes a set and dict comprehension so setup.py is parseable with Python 2.6.