packaging: add Provides: python3-mercurial and Homepage to debian package There are other packages that depend on python3-mercurial, like debian's mercurial-git, so we should mark ourselves as providing it. I compared the control file we generate to the one that the debian maintainers generate, and noticed several differences: - the Homepage bit. I included this, because why not - a more robust Suggests list that includes a graphical merge tool - a more robust Breaks list - debian's Recommends openssh-client, we only Recommends ca-certificates - a split into `mercurial` and `mercurial-common` (and possibly others?) - a slightly different description Differential Revision: https://phab.mercurial-scm.org/D9983