sparse: add a requirement when a repository uses sparse (BC) The presence of a sparse checkout can confuse legacy clients or clients without sparse enabled for reasons that should be obvious. This commit introduces a new repository requirement that tracks whether sparse is enabled. The requirement is added when a sparse config is activated and removed when the sparse config is reset. The localrepository constructor has been taught to not open repos with this requirement unless the sparse feature is enabled. It yields a more actionable error message than what you would get if the lockout were handled strictly at the requirements verification phase. Old clients that aren't sparse aware will see the generic "repository requires features unknown to this Mercurial" error, however. The new requirement has "exp" in its name to reflect the experimental nature of sparse. There's a chance that the eventual non-experimental feature won't change significantly and we could have squatted on the "sparse" requirement without ill effect. If that happens, we can teach new clients to still recognize the old name. But I suspect we'll sneak in some BC and we'll want a new requirement to convey new meaning. Differential Revision: https://phab.mercurial-scm.org/D110