posix: simplify checkexec check Use a slightly simpler logic that in some cases can avoid an unnecessary chmod and stat. Instead of flipping the X bits, make it more clear that we rely on no X bits being set on initial file creation, and that at least some of them stick after they all have been set.

posix: move checkexec test file to .hg/cache This avoids unnecessary churn in the working directory. It is not necessarily a fully valid assumption that .hg/cache is on the same filesystem as the working directory, but I think it is an acceptable approximation. It could also be the case that different parts of the working directory is on different mount points so checking in the root folder could also be wrong.

manifest: move manifestctx creation into manifestlog.get() Most manifestctx creation already happened in manifestlog.get(), but there was one spot in the manifestctx class itself that created an instance manually. This patch makes that one instance go through the manifestlog. This means extensions can just wrap manifestlog.get() and it will cover all manifestctx creations. It also means this code path now hits the manifestlog cache.