treemanifest: add configuration for using treemanifest type This change adds boolean configuration option experimental.treemanifest. When the option is enabled, manifests are parsed into the new treemanifest type. Tests can be now run using treemanifest by switching the config option default in localrepo._applyrequirements(). Tests pass even when made to randomly choose between manifestdict and treemanifest, suggesting that the two types produce identical manifests (so e.g. a manifest revlog entry written from a treemanifest can be parsed by the manifestdict code).