write_pack_file: use correct variable in diagnostic

'pack_tmp_name' is the subject of the utime() check, so report it in the
warning, not the uninitialized 'tmpname'

Signed-off-by: Sun He <sunheehnus@gmail.com>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/pack-objects.c

@@ -783,7 +783,7 @@ static void write_pack_file(void)
 				utb.modtime = --last_mtime;
 				if (utime(pack_tmp_name, &utb) < 0)
 					warning("failed utime() on %s: %s",
-						tmpname, strerror(errno));
+						pack_tmp_name, strerror(errno));
 			}
 
 			/* Enough space for "-<sha-1>.pack"? */