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/commit-msg.go

@@ -0,0 +1,64 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+package githook
+
+import (
+	"bytes"
+	"crypto/rand"
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"os/exec"
+)
+
+// AddChangeID strips comments from the commit message at args[0] and
+// prepends a random Change-Id trailer.
+//
+// Intended as a commit-msg hook.
+// https://git-scm.com/docs/githooks#_commit_msg
+func AddChangeID(args []string) error {
+	if len(args) != 1 {
+		return errors.New("usage: commit-msg message.txt")
+	}
+	file := args[0]
+	msg, err := os.ReadFile(file)
+	if err != nil {
+		return err
+	}
+	msg = filterCutLine(msg)
+
+	var id [20]byte
+	if _, err := io.ReadFull(rand.Reader, id[:]); err != nil {
+		return fmt.Errorf("could not generate Change-Id: %v", err)
+	}
+	cmdLines := [][]string{
+		{"git", "stripspace", "--strip-comments"},
+		{"git", "interpret-trailers", "--no-divider", "--where=start", "--if-exists", "doNothing", "--trailer", fmt.Sprintf("Change-Id: I%x", id)},
+	}
+	for _, cmdLine := range cmdLines {
+		if len(msg) == 0 {
+			// Don't let commands turn an empty message into a non-empty one (issue 2205).
+			break
+		}
+		cmd := exec.Command(cmdLine[0], cmdLine[1:]...)
+		cmd.Stdin = bytes.NewReader(msg)
+		msg, err = cmd.CombinedOutput()
+		if err != nil {
+			return fmt.Errorf("failed to run %v: %w\n%s", cmd, err, msg)
+		}
+	}
+	return os.WriteFile(file, msg, 0666)
+}
+
+var gitCutLine = []byte("# ------------------------ >8 ------------------------")
+
+// filterCutLine strips a `git commit -v`-style cutline and everything
+// after it from msg.
+func filterCutLine(msg []byte) []byte {
+	if before, _, ok := bytes.Cut(msg, gitCutLine); ok {
+		return before
+	}
+	return msg
+}