Whoops.  blkoff was getting set in the wrong place,
overwriting the old archive contents under `tar rf'.

Reference: /n/sources/patch/applied/tar-r-fix
Date: Sat Nov 12 21:32:41 CET 2005

--- /sys/src/cmd/tar.c	Sat Nov 12 21:32:06 2005
+++ /sys/src/cmd/tar.c	Sat Nov 12 21:32:01 2005
@@ -248,9 +248,9 @@
 
 	if (first)
 		seekable = seek(ar, 0, 1) >= 0;
+	blkoff = seek(ar, 0, 1);		/* note position for `tar r' */
 	/* try to size non-pipe input at first read */
 	if (first && usefile) {
-		blkoff = seek(ar, 0, 1);	/* note position */
 		n = read(ar, bufs, bytes);
 		if (n <= 0)
 			sysfatal("error reading archive: %r");