stream-clone: bail-out earlier if pull is partial
The `canperformstreamclone` function is bit messy. However it seems clearer to
me to process the very generic condition about "can we consider a stream-clone
at all", before checking if a stream-clone is requested and if a compatible
protocol can be negotiated with the server.
So I am doing some gratuitous movement so reorder conditional.
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true