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.