misc/git_hook: extract shared githook package; auto-rebuild on version bump (#19440)

Pull the hook logic into a reusable githook library package so
tailscale/corp can share it via a thin wrapper main instead of
keeping a forked copy in sync.

The install flow also changes: a wrapper scripts now build the
binary and reinstall the git hooks. Pulling new shared code no
longer requires re-running the installer.

Updates tailscale/corp#39860

Change-Id: I4d606d11c8c883015c190c54e3387a7f9fe4dd32

Signed-off-by: Fernando Serboncini <fserb@tailscale.com>

misc/git_hook/githook/githook.go

@@ -0,0 +1,46 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package githook contains the shared implementation of Tailscale's git
+// hooks. The tailscale/tailscale and tailscale/corp repositories each have
+// a thin main package that dispatches to this one, calling individual
+// hook functions with per-repo arguments as needed.
+package githook
+
+import (
+	_ "embed"
+	"errors"
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+)
+
+// Launcher is the canonical bytes of launcher.sh. Downstream repos
+// (e.g. tailscale/corp) rely on these bytes at install time.
+//
+//go:embed launcher.sh
+var Launcher []byte
+
+// RunLocalHook runs an optional user-supplied hook at
+// .git/hooks/<name>.local, if present.
+func RunLocalHook(hookName string, args []string) error {
+	cmdPath, err := os.Executable()
+	if err != nil {
+		return err
+	}
+	localHookPath := filepath.Join(filepath.Dir(cmdPath), hookName+".local")
+	if _, err := os.Stat(localHookPath); errors.Is(err, os.ErrNotExist) {
+		return nil
+	} else if err != nil {
+		return fmt.Errorf("checking for local hook: %w", err)
+	}
+
+	cmd := exec.Command(localHookPath, args...)
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	if err := cmd.Run(); err != nil {
+		return fmt.Errorf("running local hook %q: %w", localHookPath, err)
+	}
+	return nil
+}