Bug#570962: Patch for apt-mark that shows usage when no argument is provided

[Torsten Spindler]

The attached patch shows the usage information when no argument is given
to apt-mark.

--- apt-mark.orig	2010-03-04 16:23:23.450331213 +0100
+++ apt-mark	2010-03-04 16:27:30.974347744 +0100
@@ -82,6 +82,9 @@
     else:
         STATE_FILE=options.filename
 
+    if len(args) == 0:
+        parser.error("first argument must be 'markauto', 'unmarkauto' or 'showauto'")
+
     if args[0] == "showauto":
         show_automatic(STATE_FILE)
     else: