manifest: change treemanifestctx to construct subtrees from the manifestlog Previously, treemanifestctx would directly construct its subtrees. By making it get the subtrees through manifestlog.get() we consolidate all treemanifestctx creation into manifestlog.get() and therefore extensions that need to wrap manifestctx creation (like narrow-hg) can intercept manifestctxs at that single place. This also means fetching subtrees will take advantage of the manifestlog ctx cache now, which it did not before.

manifest: make revlog verification optional This patches adds an parameter to manifestlog.get() to disable hash checking. This will be used in an upcoming patch to support treemanifestctx reading sub-trees without loading them from the revlog. (This is already supported but does not go through the manifestlog.get() code path)