rust-cpython: binding for headrevs() This uses the core `dagops::retain_heads` to give a Rust implementation to `mercurial.dagop.headrevs`. Testing happens for now from `test-rust-ancestors.py` (for quick and minimal change), but it'd made more sense to put the binary index data elsewhere and to create a new test python module

rust-cpython: moved py_set() utility to conversion module We're still hoping to get rid of it eventually, but we're going to need it from outside the `ancestors` module before that.

rust: translated random test of missingancestors This is a Rust implementation of the random DAG generator and related incrementalmissingancestors tests against a naive brute force implementation. It is provided as an integration test, so that it won't run by default if any unit test fails. In case of a failed example, all needed information for reproduction is included in the panic message, (this is how `test_remove_ancestors_from_case1()` has been generated), as well as the random seed. The whole test is rerunnable by passing the random seed in the TEST_RANDOM_SEED environment variable. The other parameters (numbers of iterations) can be passed in the TEST_MISSING_ANCESTORS environment variable. An alternative would have been to expose to Python MissingAncestors<VecGraphs> but that would have meant pollution of the release build used from Python, whereas we do it in this changeset within the tests submodule Differential Revision: https://phab.mercurial-scm.org/D5417

debugpathcopies: fix typo in synopsis Differential Revision: https://phab.mercurial-scm.org/D5949

debugrename: don't require at least one path I don't see a reason that it needs to require a path. Most commands match everything when no paths are given, but here you have to do something like `hg debugrename -r . .` (from the repo root) to match everything. Differential Revision: https://phab.mercurial-scm.org/D5948

revlog: use iterbytestr() Otherwise we iterate over integers in Python 3 and the character compare fails. Differential Revision: https://phab.mercurial-scm.org/D5947