memctx: allow the metadataonlyctx thats reusing the manifest node When we have a lot of files writing a new manifest revision can be expensive. This commit adds a possibility for memctx to reuse a manifest from a different commit. This can be beneficial for commands that are creating metadata changes without any actual files changed like "hg metaedit" in evolve extension. I will send the change for evolve that leverages this once this is accepted.