Remove log filtering and add stringsetflag.

common/flag/stringsetflag/stringsetflag.go

Index: common/flag/stringsetflag/stringsetflag.go
diff --git a/common/flag/stringsetflag/stringsetflag.go b/common/flag/stringsetflag/stringsetflag.go
new file mode 100644
index 0000000000000000000000000000000000000000..bec866a2dcd12632651be197923a1510ed6256d7
--- /dev/null
+++ b/common/flag/stringsetflag/stringsetflag.go
@@ -0,0 +1,52 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Package stringsetflag provides a flag.Value implementation which resolves
+// multiple args into a stringset.
+package stringsetflag
+
+import (
+	"flag"
+	"fmt"
+	"sort"
+	"strings"
+
+	"github.com/luci/luci-go/common/stringset"
+)
+
+// Flag is a flag.Value implementation which represents an unordered set of
+// strings.
+//
+// For example, this allows you to construct a flag that would behave like:
+//   -myflag Foo
+//   -myflag Bar
+//   -myflag Bar
+//
+// And then myflag.Data.Has("Bar") would be true.
+type Flag struct{ Data stringset.Set }
+
+var _ flag.Value = (*Flag)(nil)
+
+func (f Flag) String() string {
+	if f.Data == nil {
+		return ""
+	}
+	slc := f.Data.ToSlice()
+	sort.Strings(slc)
+	return strings.Join(slc, ",")
+}
+
+// Set implements flag.Value's Set function.
+func (f *Flag) Set(val string) error {
+	if val == "" {
+		return fmt.Errorf("must have an argument value")
+	}
+
+	if f.Data == nil {
+		f.Data = stringset.NewFromSlice(val)
+	} else {
+		f.Data.Add(val)
+	}
+	return nil
+}