receive-pack: respect receive.advertiseAlternates config

Usually receive-pack advertises ref tips from alternates
repositories so that clients can sometimes avoid sending
objects that are already upstream.

However, if you have a very large alternates network, then
the number of ".have" refs can get cumbersome, and you spend
more time advertising refs that the client doesn't care
about than you are saving in the optimization.

This patch adds a config variable to drop .have lines
entirely.

An alternative approach would be to restrict the .have lines
to a smaller portion of the namespace that is known to be
"interesting" to most clients. This patch is much simpler;
if the loss of the .have optimization turns out to be too
much, we can try something more complex.