fetch-pack: grow stateless RPC windows exponentially

fetch-pack: grow stateless RPC windows exponentially

When updating large repositories, the LARGE_FLUSH limit (that is, the
limit at which the window growth strategy switches from exponential to
linear) is reached quite quickly. Use a conservative exponential growth
strategy when that limit is reached instead (and increase LARGE_FLUSH so
that there is no regression in window size).

This optimization is only applied during stateless RPCs to avoid the
issue raised and fixed in commit 44d8dc54 (Fix potential local
deadlock during fetch-pack, 2011-03-29).

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