cmd/tailscale: add shell tab-completion

The approach is lifted from cobra: `tailscale completion bash` emits a bash
script for configuring the shell's autocomplete:

    . <( tailscale completion bash )

so that typing:

    tailscale st<TAB>

invokes:

    tailscale completion __complete -- st

RELNOTE=tailscale CLI now supports shell tab-completion

Fixes #3793

Signed-off-by: Paul Scott <paul@tailscale.com>

tempfork/spf13/cobra/gen.go

@@ -0,0 +1,42 @@
+//go:build gen
+
+package main
+
+import (
+	"compress/gzip"
+	"fmt"
+	"io"
+	"os"
+)
+
+func main() {
+	for _, name := range []string{"comp.bash", "comp.zsh", "comp.fish", "comp.ps1"} {
+		err := compress(name)
+		if err != nil {
+			fmt.Fprintln(os.Stderr, "compressing "+name+":", err)
+			os.Exit(1)
+		}
+	}
+}
+
+func compress(name string) error {
+	src, err := os.Open(name)
+	if err != nil {
+		return err
+	}
+	defer src.Close()
+
+	dst, err := os.Create(name + ".gz")
+	if err != nil {
+		return err
+	}
+	defer dst.Close()
+
+	z := gzip.NewWriter(dst)
+	_, err = io.Copy(z, src)
+	if err != nil {
+		return err
+	}
+
+	return z.Close()
+}