exchange: support for streaming clone bundles Now that we have a mechanism to produce and consume streaming clone bundles, we need to teach the human-facing bundle specification parser and the internal bundle file header reading code to be aware of this new format. This patch does so. For the human-facing bundle specification, we choose the name "packed" to describe "streaming clone bundles" because the bundle is essentially a "pack" of raw revlog files that are "packed" together. There should probably be a bikeshed over the name, especially since it is human facing.