Refactor SideBandInputStream construction

Typically we refer to the raw InputStream (the stream without the
pkt-line headers on it) as rawIn, and the pkt-line header variant
as pckIn.  Refactor our fields to reflect that.  To ensure these
are actually the same underlying InputStream, we now create our own
PacketLineIn wrapper around the supplied raw InputStream.  Its a
very low-cost object since it has only the 4 byte length buffer.

Instead of hardcoding the header length as 5, use the constant from
SideBandOutputStream.  This makes it a bit more clear what we are
consuming, exactly here.

Change-Id: Iebd05538042913536b88c3ddc3adc3a86a841cc5
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>