git-pack-objects: Automatically pack annotated tags if object was packed

git-pack-objects: Automatically pack annotated tags if object was packed

The new option "--include-tag" allows the caller to request that
any annotated tag be included into the packfile if the object the tag
references was also included as part of the packfile.

This option can be useful on the server side of a native git transport,
where the server knows what commits it is including into a packfile to
update the client.  If new annotated tags have been introduced then we
can also include them in the packfile, saving the client from needing
to request them through a second connection.

This change only introduces the backend option and provides a test.
Protocol extensions to make this useful in fetch-pack/upload-pack
are still necessary to activate the logic during transport.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>