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.
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Ran 16 tests in 0.000s
OK