receive-pack.c: negotiate atomic push support

receive-pack.c: negotiate atomic push support

This adds the atomic protocol option to allow
receive-pack to inform the client that it has
atomic push capability.

This commit makes the functionality introduced
in the previous commits go live for the serving
side. The changes in documentation reflect the
protocol capabilities of the server.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>