signed push: add "pushee" header to push certificate

Record the URL of the intended recipient for a push (after
anonymizing it if it has authentication material) on a new "pushee
URL" header.  Because the networking configuration (SSH-tunnels,
proxies, etc.) on the pushing user's side varies, the receiving
repository may not know the single canonical URL all the pushing
users would refer it as (besides, many sites allow pushing over
ssh://host/path and https://host/path protocols to the same
repository but with different local part of the path).  So this
value may not be reliably used for replay-attack prevention
purposes, but this will still serve as a human readable hint to
identify the repository the certificate refers to.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

send-pack.h

@@ -2,6 +2,7 @@
 #define SEND_PACK_H
 
 struct send_pack_args {
+	const char *url;
 	unsigned verbose:1,
 		quiet:1,
 		porcelain:1,