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/comp.go

@@ -0,0 +1,72 @@
+package cobra
+
+import (
+	"bytes"
+	"compress/gzip"
+	_ "embed"
+	"fmt"
+	"io"
+)
+
+//go:generate go run gen.go
+
+//go:embed comp.bash.gz
+var compBash string
+
+func ScriptBash(w io.Writer, name, compCmd, nameForVar string) error {
+	return fmtgz(
+		w, compBash,
+		name, compCmd,
+		ShellCompDirectiveError, ShellCompDirectiveNoSpace, ShellCompDirectiveNoFileComp,
+		ShellCompDirectiveFilterFileExt, ShellCompDirectiveFilterDirs, ShellCompDirectiveKeepOrder,
+	)
+}
+
+//go:embed comp.zsh.gz
+var compZsh string
+
+func ScriptZsh(w io.Writer, name, compCmd, nameForVar string) error {
+	return fmtgz(
+		w, compZsh,
+		name, compCmd,
+		ShellCompDirectiveError, ShellCompDirectiveNoSpace, ShellCompDirectiveNoFileComp,
+		ShellCompDirectiveFilterFileExt, ShellCompDirectiveFilterDirs, ShellCompDirectiveKeepOrder,
+	)
+}
+
+//go:embed comp.fish.gz
+var compFish string
+
+func ScriptFish(w io.Writer, name, compCmd, nameForVar string) error {
+	return fmtgz(
+		w, compFish,
+		nameForVar, name, compCmd,
+		ShellCompDirectiveError, ShellCompDirectiveNoSpace, ShellCompDirectiveNoFileComp,
+		ShellCompDirectiveFilterFileExt, ShellCompDirectiveFilterDirs, ShellCompDirectiveKeepOrder,
+	)
+}
+
+//go:embed comp.ps1.gz
+var compPowershell string
+
+func ScriptPowershell(w io.Writer, name, compCmd, nameForVar string) error {
+	return fmtgz(
+		w, compPowershell,
+		name, nameForVar, compCmd,
+		ShellCompDirectiveError, ShellCompDirectiveNoSpace, ShellCompDirectiveNoFileComp,
+		ShellCompDirectiveFilterFileExt, ShellCompDirectiveFilterDirs, ShellCompDirectiveKeepOrder,
+	)
+}
+
+func fmtgz(w io.Writer, formatgz string, args ...any) error {
+	f, err := gzip.NewReader(bytes.NewBufferString(formatgz))
+	if err != nil {
+		return fmt.Errorf("decompressing script: %w", err)
+	}
+	format, err := io.ReadAll(f)
+	if err != nil {
+		return fmt.Errorf("decompressing script: %w", err)
+	}
+	_, err = fmt.Fprintf(w, string(format), args...)
+	return err
+}