fetch-pack: parse and advertise the object-format capability

Parse the server's object-format capability and respond accordingly,
dying if there is a mismatch.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fetch-pack.c

@@ -1179,6 +1179,7 @@ static int send_fetch_request(struct fetch_negotiator *negotiator, int fd_out,
 			      int sideband_all, int seen_ack)
 {
 	int ret = 0;
+	const char *hash_name;
 	struct strbuf req_buf = STRBUF_INIT;
 
 	if (server_supports_v2("fetch", 1))
@@ -1193,6 +1194,17 @@ static int send_fetch_request(struct fetch_negotiator *negotiator, int fd_out,
 					 args->server_options->items[i].string);
 	}
 
+	if (server_feature_v2("object-format", &hash_name)) {
+		int hash_algo = hash_algo_by_name(hash_name);
+		if (hash_algo_by_ptr(the_hash_algo) != hash_algo)
+			die(_("mismatched algorithms: client %s; server %s"),
+			    the_hash_algo->name, hash_name);
+		packet_write_fmt(fd_out, "object-format=%s", the_hash_algo->name);
+	} else if (hash_algo_by_ptr(the_hash_algo) != GIT_HASH_SHA1) {
+		die(_("the server does not support algorithm '%s'"),
+		    the_hash_algo->name);
+	}
+
 	packet_buf_delim(&req_buf);
 	if (args->use_thin_pack)
 		packet_buf_write(&req_buf, "thin-pack");