fetch-pack: make negotiation-related vars local

fetch-pack: make negotiation-related vars local

Reduce the number of global variables by making the priority queue and
the count of non-common commits in it local, passing them as a struct to
various functions where necessary.

This also helps in the case that fetch_pack() is invoked twice in the
same process (when tag following is required when using a transport that
does not support tag following), in that different priority queues will
now be used in each invocation, instead of reusing the possibly
non-empty one.

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