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>

Documentation/technical/pack-protocol.txt

@@ -484,6 +484,7 @@ references.
   push-cert         = PKT-LINE("push-cert" NUL capability-list LF)
 		      PKT-LINE("certificate version 0.1" LF)
 		      PKT-LINE("pusher" SP ident LF)
+		      PKT-LINE("pushee" SP url LF)
 		      PKT-LINE(LF)
 		      *PKT-LINE(command LF)
 		      *PKT-LINE(gpg-signature-lines LF)
@@ -527,6 +528,11 @@ Currently, the following header fields are defined:
 	Identify the GPG key in "Human Readable Name <email@address>"
 	format.
 
+`pushee` url::
+	The repository URL (anonymized, if the URL contains
+	authentication material) the user who ran `git push`
+	intended to push into.
+
 The GPG signature lines are a detached signature for the contents
 recorded in the push certificate before the signature block begins.
 The detached signature is used to certify that the commands were

send-pack.c

@@ -240,6 +240,11 @@ static int generate_push_cert(struct strbuf *req_buf,
 	datestamp(stamp, sizeof(stamp));
 	strbuf_addf(&cert, "certificate version 0.1\n");
 	strbuf_addf(&cert, "pusher %s %s\n", signing_key, stamp);
+	if (args->url && *args->url) {
+		char *anon_url = transport_anonymize_url(args->url);
+		strbuf_addf(&cert, "pushee %s\n", anon_url);
+		free(anon_url);
+	}
 	strbuf_addstr(&cert, "\n");
 
 	for (ref = remote_refs; ref; ref = ref->next) {

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,

transport.c

@@ -827,6 +827,7 @@ static int git_transport_push(struct transport *transport, struct ref *remote_re
 	args.dry_run = !!(flags & TRANSPORT_PUSH_DRY_RUN);
 	args.porcelain = !!(flags & TRANSPORT_PUSH_PORCELAIN);
 	args.push_cert = !!(flags & TRANSPORT_PUSH_CERT);
+	args.url = transport->url;
 
 	ret = send_pack(&args, data->fd, data->conn, remote_refs,
 			&data->extra_have);