streamclone: move "streaming all changes" message location Previously, the message was printed after we requested and started processing the remote stream. This seems like something that we should do before calling out to the remote. Moving it also makes it easier to deal with the bundle2 implementation.

streamclone: move payload header generation into own function The stream clone data over the wire protocol contains a header line indicating total file count and data size. In bundle2, this metadata can be captured by a part parameter and doesn't need to be in the body. In preparation for bundle2, have generatev1() return the raw metadata and move the header generation to its own function.