manifest: change manifestlog mancache to be directory based In the last patch we added a get() function that allows fetching directory level treemanifestctxs. It didn't handle caching at directory level though, so we need to change our mancache to support multiple directories.

manifest: add manifestlog.get to obtain subdirectory instances Previously manifestlog only allowed obtaining root level manifests. Future patches will need direct access to subdirectory manifests as part of changegroup creation, so let's add a get() function that knows how to deal with subdirectories.

manifest: throw LookupError if node not in revlog When accessing a manifest via manifestlog[node], let's verify that the node actually exists and throw a LookupError if it doesn't. This matches the old read behavior, so we don't accidentally return invalid manifestctxs. We do this in manifestlog instead of in the manifestctx/treemanifestctx constructors because the treemanifest code currently relies on the fact that certain code paths can produce treemanifests without touching the revlogs (and it has tests that verify things work if certain revlogs are missing entirely, so they break if we add validation that tries to read them).