[vpython] Fix flag parsing bug with "--".

vpython/application/flag_test.go

 // Copyright 2017 The LUCI Authors. All rights reserved.
 // Use of this source code is governed under the Apache License, Version 2.0
 // that can be found in the LICENSE file.
 
 package application
 
 import (
         "flag"
         "fmt"
         "testing"
@@ skipping 30 matching lines @@
                                 []string{"-i", "-log-level", "debug"},
                                 []string{},
                                 []string{"-i", "-log-level", "debug"},
                         },
 
                         {
                                 []string{"-log-level", "--", "ohai"},
                                 []string{"-log-level"},
                                 []string{"--", "ohai"},
                         },
+
+                        {
+                                []string{"--log-level", "debug", "-d", "--", "script"},
+                                []string{"--log-level", "debug"},
+                                []string{"-d", "--", "script"},
+                        },
                 } {
                         Convey(fmt.Sprintf(`Flags %v are split into %v and %v`, tc.args, tc.self, tc.extra), func() {
                                 self, extra := extractFlagsForSet(tc.args, fs)
                                 So(self, ShouldResemble, tc.self)
                                 So(extra, ShouldResemble, tc.extra)
                         })
                 }
         })
 }