Update packfile transfer protocol documentation

Update packfile transfer protocol documentation

The current technical documentation for the packfile protocol is both
sparse and incorrect.  This documents the fetch-pack/upload-pack and
send-pack/ receive-pack protocols much more fully.

Add documentation from Shawn's upcoming http-protocol docs that is
shared by the packfile protocol. protocol-common.txt describes ABNF
notation amendments, refname rules and the packet line format.

Add documentation on the various capabilities supported by the
upload-pack and receive-pack protocols. protocol-capabilities.txt
describes multi-ack, thin-pack, side-band[-64k], shallow, no-progress,
include-tag, ofs-delta, delete-refs and report-status.

Signed-off-by: Scott Chacon <schacon@gmail.com>
Signed-off-by: Nanako Shiraishi <nanako3@lavabit.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>