push: support signing pushes iff the server supports it

push: support signing pushes iff the server supports it

Add a new flag --sign=true (or --sign=false), which means the same
thing as the original --signed (or --no-signed).  Give it a third
value --sign=if-asked to tell push and send-pack to send a push
certificate if and only if the server advertised a push cert nonce.

If not, warn the user that their push may not be as secure as they
thought.

Signed-off-by: Dave Borowitz <dborowitz@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>