tests: add test to demonstrate issue6159 Differential Revision: https://phab.mercurial-scm.org/D6740

packaging: add Bullseye, remove Jessie Jessie is oldoldstable now, and Bullseye is going to be the next stable (now testing). We're continuing to support the current oldstable, stable and testing releases. Differential Revision: https://phab.mercurial-scm.org/D6762

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- Cosmic was publicly supported until 2019-07-18 - Disco will be publicly supported until 2020-01 Cosmic is officially out-of-date, but since it still may be in use, and because we didn't add it when it first came out, I think it would be nice to support it until the next time somebody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.