Teach UploadPack how to use an RPC style interface

If biDirectionalPipe is false UploadPack does not start out with
the advertisement but instead assumes it should read one block of
want/have lines, process that, and write the ACK/NAKs out.

This means it only is doing one read through the input followed by
one write to the output, which fits with the HTTP request processing
model, and any other type of RPC system.

Change-Id: Ia9f7c46ee556f996367180f15d2caa8572cdd59f
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>