Add cipd selfupdate command.

cipd/client/cipd/install_client.go

Index: cipd/client/cipd/install_client.go
diff --git a/cipd/client/cipd/install_client.go b/cipd/client/cipd/install_client.go
new file mode 100644
index 0000000000000000000000000000000000000000..947ecd14894f9009f740aaf0e50c507805c7595b
--- /dev/null
+++ b/cipd/client/cipd/install_client.go
@@ -0,0 +1,29 @@
+// Copyright 2015 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.
+
+// +build !windows
+
+package cipd
+
+import (
+	"os"
+
+	"github.com/luci/luci-go/cipd/client/cipd/local"
+	"golang.org/x/net/context"
+)
+
+func (client *clientImpl) installClient(ctx context.Context, fs local.FileSystem, fetchURL, destination string) error {
+	curStat, err := os.Stat(destination)
+	if err != nil {
+		return err
+	}
+
+	return fs.EnsureFile(ctx, destination, func(of *os.File) error {
+		if err := of.Chmod(curStat.Mode()); err != nil {
+			return err
+		}
+		// TODO(iannucci): worry about owner/group?
+		return client.storage.download(ctx, fetchURL, of)
+	})
+}