pkt-line: add delim packet support

pkt-line: add delim packet support

One of the design goals of protocol-v2 is to improve the semantics of
flush packets.  Currently in protocol-v1, flush packets are used both to
indicate a break in a list of packet lines as well as an indication that
one side has finished speaking.  This makes it particularly difficult
to implement proxies as a proxy would need to completely understand git
protocol instead of simply looking for a flush packet.

To do this, introduce the special deliminator packet '0001'.  A delim
packet can then be used as a deliminator between lists of packet lines
while flush packets can be reserved to indicate the end of a response.

Documentation for how this packet will be used in protocol v2 will
included in a future patch.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>