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.

streamclone: move payload header line consumption bundle2 parts have parameters. These are a logical place for "header" data such as the file count and payload size of stream clone data. In preparation for supporting stream clones with bundle2, move the consumption of the header line from the payload into maybeperformlegacystreamclone(). Note: the header line is still being emitted by generatev1(). This will be addressed in a subsequent patch.