xflag: Add counting flag from go.git internals

Because not only go.git wants to increase verbosity on `-v -v ...`.

Code taken from go.git ec80737bdf.

LICENSE-go

+Copyright (c) 2009 The Go Authors. All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are
+met:
+
+   * Redistributions of source code must retain the above copyright
+notice, this list of conditions and the following disclaimer.
+   * Redistributions in binary form must reproduce the above
+copyright notice, this list of conditions and the following disclaimer
+in the documentation and/or other materials provided with the
+distribution.
+   * Neither the name of Google Inc. nor the names of its
+contributors may be used to endorse or promote products derived from
+this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

xflag/doc.go

+// Package xflag provides addons to standard package flag
+package xflag

xflag/xflag.go

+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE-go file.
+
+package xflag
+
+import (
+	"fmt"
+	"strconv"
+)
+
+// originally taken from cmd/dist.count in go.git
+
+// Count is a flag.Value that is like a flag.Bool and a flag.Int.
+// If used as -name, it increments the Count, but -name=x sets the Count.
+// Used for verbose flag -v.
+type Count int
+
+func (c *Count) String() string {
+	return fmt.Sprint(int(*c))
+}
+
+func (c *Count) Set(s string) error {
+	switch s {
+	case "true":
+		*c++
+	case "false":
+		*c = 0
+	default:
+		n, err := strconv.Atoi(s)
+		if err != nil {
+			return fmt.Errorf("invalid count %q", s)
+		}
+		*c = Count(n)
+	}
+	return nil
+}
+
+func (c *Count) IsBoolFlag() bool {
+	return true
+}