upload-pack: add object filtering for partial clone

upload-pack: add object filtering for partial clone

Teach upload-pack to negotiate object filtering over the protocol and
to send filter parameters to pack-objects.  This is intended for partial
clone and fetch.

The idea to make upload-pack configurable using uploadpack.allowFilter
comes from Jonathan Tan's work in [1].

[1] https://public-inbox.org/git/f211093280b422c32cc1b7034130072f35c5ed51.1506714999.git.jonathantanmy@google.com/

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>