{fetch,upload}-pack: support filter in protocol v2

{fetch,upload}-pack: support filter in protocol v2

The fetch-pack/upload-pack protocol v2 was developed independently of
the filter parameter (used in partial fetches), thus it did not include
support for it. Add support for the filter parameter.

Like in the legacy protocol, the server advertises and supports "filter"
only if uploadpack.allowfilter is configured.

Like in the legacy protocol, the client continues with a warning if
"--filter" is specified, but the server does not advertise it.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>