treemanifest: allow manifestrevlog to take an explicit treemanifest arg Previously we relied on the opener options to tell the revlog to be a tree manifest. This makes it complicated for extensions to create treemanifests and normal manifests at the same time. Let's add a construtor argument to create a treemanifest revlog as well. I considered removing the options['treemanifest'] logic from manifestrevlog entirely, but doing so shifts the responsibility to the caller which ends up requiring changes in localrepo, bundlerepo, and unionrepo. I figured having the dual mechanism was better than polluting other parts of the code base with treemanifest knowledge.