make the sender advertise shallow commits to the receiver

If either receive-pack or upload-pack is called on a shallow
repository, shallow commits (*) will be sent after the ref
advertisement (but before the packet flush), so that the receiver has
the full "shape" of the sender's commit graph. This will be needed for
the receiver to update its .git/shallow if necessary.

This breaks the protocol for all clients trying to push to a shallow
repo, or fetch from one. Which is basically the same end result as
today's "is_repository_shallow() && die()" in receive-pack and
upload-pack. New clients will be made aware of shallow upstream and
can make use of this information.

The sender must send all shallow commits that are sent in the
following pack. It may send more shallow commits than necessary.

upload-pack for example may choose to advertise no shallow commits if
it knows in advance that the pack it's going to send contains no
shallow commits. But upload-pack is the server, so we choose the
cheaper way, send full .git/shallow and let the client deal with it.

Smart HTTP is not affected by this patch. Shallow support on
smart-http comes later separately.

(*) A shallow commit is a commit that terminates the revision
    walker. It is usually put in .git/shallow in order to keep the
    revision walker from going out of bound because there is no
    guarantee that objects behind this commit is available.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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