treemanifest: make node reuse match flat manifest behavior In a flat manifest, a node with the same content but different parents is still considered a new node. In the current tree manifests however, if the content is the same, we ignore the parents entirely and just reuse the existing node. In our external treemanifest extension, we want to allow having one treemanifest for every flat manifests, as a way of easeing the migration to treemanifests. To make this possible, let's change the root node treemanifest behavior to match the behavior for flat manifests, so we can have a 1:1 relationship. While this sounds like a BC breakage, it's not actually a state users can normally get in because: A) you can't make empty commits, and B) even if you try to make an empty commit (by making a commit then amending it's changes away), the higher level commit logic in localrepo.commitctx() forces the commit to use the original p1 manifest node if no files were changed. So this would only affect extensions and automation that reached passed the normal localrepo.commit() logic straight into the manifest logic.