http: replace hard-coded constant with the_hash_algo

Replace a hard-coded 40 with a reference to the_hash_algo.

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

http.c

@@ -2065,7 +2065,7 @@ int http_fetch_ref(const char *base, struct ref *ref)
 	url = quote_ref_url(base, ref->name);
 	if (http_get_strbuf(url, &buffer, &options) == HTTP_OK) {
 		strbuf_rtrim(&buffer);
-		if (buffer.len == 40)
+		if (buffer.len == the_hash_algo->hexsz)
 			ret = get_oid_hex(buffer.buf, &ref->old_oid);
 		else if (starts_with(buffer.buf, "ref: ")) {
 			ref->symref = xstrdup(buffer.buf + 5);