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

sparse: consolidate common code for writing sparse config In 3 functions we were writing the sparse config and updating the working directory. In two of them we had a transaction-like process for restoring the sparse config in case of wdir update fail. Because the pattern is common, we've already made mistakes, and the complexity will increase in the near future, let's consolidate the code into a reusable function. As part of this refactor, we end up reading the "sparse" file twice when updating it. This is a bit sub-optimal. But I don't think it is worth the code complexity to pass around the variables to avoid the redundancy. Differential Revision: https://phab.mercurial-scm.org/D109