for_each_alternate_ref: use strbuf for path allocation

We have a string with ".../objects" pointing to the
alternate object store, and overwrite bits of it to look at
other paths in the (potential) git repository holding it.
This works because the only path we care about is "refs",
which is shorter than "objects".

Using a strbuf to hold the path lets us get rid of some
magic numbers, and makes it more obvious that the memory
operations are safe.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

transport.c

@@ -1214,34 +1214,34 @@ struct alternate_refs_data {
 static int refs_from_alternate_cb(struct alternate_object_database *e,
 				  void *data)
 {
-	char *other;
-	size_t len;
+	struct strbuf path = STRBUF_INIT;
+	size_t base_len;
 	struct remote *remote;
 	struct transport *transport;
 	const struct ref *extra;
 	struct alternate_refs_data *cb = data;
 
-	other = real_pathdup(e->path);
-	if (!other)
-		return 0;
-	len = strlen(other);
-
-	if (len < 8 || memcmp(other + len - 8, "/objects", 8))
+	if (!strbuf_realpath(&path, e->path, 0))
+		goto out;
+	if (!strbuf_strip_suffix(&path, "/objects"))
 		goto out;
+	base_len = path.len;
+
 	/* Is this a git repository with refs? */
-	memcpy(other + len - 8, "/refs", 6);
-	if (!is_directory(other))
+	strbuf_addstr(&path, "/refs");
+	if (!is_directory(path.buf))
 		goto out;
-	other[len - 8] = '\0';
-	remote = remote_get(other);
-	transport = transport_get(remote, other);
+	strbuf_setlen(&path, base_len);
+
+	remote = remote_get(path.buf);
+	transport = transport_get(remote, path.buf);
 	for (extra = transport_get_remote_refs(transport);
 	     extra;
 	     extra = extra->next)
 		cb->fn(extra, cb->data);
 	transport_disconnect(transport);
 out:
-	free(other);
+	strbuf_release(&path);
 	return 0;
 }