version: use OSS repo's version when building.

When building with redo, also include the git commit hash
from the proprietary repo, so that we have a precise commit
that identifies all build info (including Go toolchain version).

Add a top-level build script demonstrating to downstream distros
how to burn the right information into builds.

Adjust `tailscale version` to print commit hashes when available.

Fixes #841.

Signed-off-by: David Anderson <danderson@tailscale.com>

version/print.go

@@ -0,0 +1,25 @@
+// Copyright (c) 2020 Tailscale Inc & 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 version
+
+import (
+	"fmt"
+	"runtime"
+	"strings"
+)
+
+func String() string {
+	var ret strings.Builder
+	ret.WriteString(Short)
+	ret.WriteByte('\n')
+	if GitCommit != "" {
+		fmt.Fprintf(&ret, "  tailscale commit: %s\n", GitCommit)
+	}
+	if ExtraGitCommit != "" {
+		fmt.Fprintf(&ret, "  other commit: %s\n", ExtraGitCommit)
+	}
+	fmt.Fprintf(&ret, "  go version: %s\n", runtime.Version())
+	return strings.TrimSpace(ret.String())
+}