packaging: add Cosmic and Disco, remove Trusty and Artful - Trusty was publicly supported until 2019-04-30 - Artful was publicly supported until 2018-07-19 - Cosmic was publicly supported until 2019-07-18 - Disco will be publicly supported until 2020-01 Cosmic is officially out-of-date, but since it still may be in use, and because we didn't add it when it first came out, I think it would be nice to support it until the next time somebody decides to update this list of Ubuntu releases. Differential Revision: https://phab.mercurial-scm.org/D6761

narrow: fix typo "respositories" Differential Revision: https://phab.mercurial-scm.org/D6758

merge with stable

makefile: run Rust tests if cargo is installed While no particular minimum toolchain version is targeted as of yet, this serves as a first step to make more people/machines run the Rust tests.

merge: hint about using `hg resolve` for resolving conflicts This was suggested by one of our users at Google. Makes sense to me. Differential Revision: https://phab.mercurial-scm.org/D6755

rust-dirstate: remove test case for DirsMultiset::new(Manifest, Some) It's no longer possible.

rust-dirstate: split DirsMultiset constructor per input type Since skip_state only applies to dirstate, it doesn't make sense to unify these constructors and dispatch by enum.

rust-dirstate: remove excessive clone() of parameter and return value I think pass-by-ref is preferred in general.

rust-dirstate: handle invalid length of p1/p2 parameters It uses match syntax since map_err() failed to deduce the argument type.

rust: simply use TryInto to convert slice to array Since our rust module depends on TryInto, there's no point to avoid using it. While rewriting copy_into_array(), I noticed CPython interface doesn't check the length of the p1/p2 values, which is marked as TODO.