push: add '--prune' option

When pushing groups of refs to a remote, there is no simple way to remove
old refs that still exist at the remote that is no longer updated from us.
This will allow us to remove such refs from the remote.

With this change, running this command

$ git push --prune remote refs/heads/*:refs/remotes/laptop/*

removes refs/remotes/laptop/foo from the remote if we do not have branch
"foo" locally anymore.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>