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