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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https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
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commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).