changegroup: write root manifests and subdir manifests in a single loop This is another step towards making the manifest generation recurse along the directory trees. The loop over 'tmfnodes' now takes the form of a queue. At this point, we only add to the queue twice: we add the root manifests, and, while visiting the root manifest revisions, we add all subdirectory revisions (for treemanifest repos). Thus, any iterations over 'tmfnodes' after the first will not add any items and the "queue" will just keep shrinking.