streamclone: move payload header line consumption...
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.
