tar: fix a thinko in prev commit - we need to copy to _unused_ fd

function                                             old     new   delta
vfork_compressor                                     257     246     -11

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/archival/tar.c b/archival/tar.c
index 120c77f..1c71f7f 100644
--- a/archival/tar.c
+++ b/archival/tar.c
@@ -632,8 +632,7 @@
 			 * Swapping move_fd's order wouldn't work:
 			 * data.rd is 1 and _it_ would be destroyed.
 			 */
-			xmove_fd(tfd, 3);
-			tfd = 3;
+			tfd = dup(tfd);
 		}
 		xmove_fd(data.rd, 0);
 		xmove_fd(tfd, 1);