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/pre-commit.go

@@ -0,0 +1,62 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+package githook
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"os/exec"
+
+	"github.com/fatih/color"
+	"github.com/sourcegraph/go-diff/diff"
+)
+
+var preCommitForbiddenPatterns = [][]byte{
+	// Concatenation avoids tripping the check on this file.
+	[]byte("NOCOM" + "MIT"),
+	[]byte("DO NOT " + "SUBMIT"),
+}
+
+// CheckForbiddenMarkers scans the staged diff for forbidden markers
+// and returns an error if any are found.
+//
+// Intended as a pre-commit hook.
+// https://git-scm.com/docs/githooks#_pre_commit
+func CheckForbiddenMarkers() error {
+	diffOut, err := exec.Command("git", "diff", "--cached").Output()
+	if err != nil {
+		return fmt.Errorf("could not get git diff: %w", err)
+	}
+
+	diffs, err := diff.ParseMultiFileDiff(diffOut)
+	if err != nil {
+		return fmt.Errorf("could not parse diff: %w", err)
+	}
+
+	foundForbidden := false
+	for _, d := range diffs {
+		for _, hunk := range d.Hunks {
+			lines := bytes.Split(hunk.Body, []byte{'\n'})
+			for i, line := range lines {
+				if len(line) == 0 || line[0] != '+' {
+					continue
+				}
+				for _, forbidden := range preCommitForbiddenPatterns {
+					if bytes.Contains(line, forbidden) {
+						if !foundForbidden {
+							color.New(color.Bold, color.FgRed, color.Underline).Printf("%s found:\n", forbidden)
+						}
+						fmt.Printf("%s:%d: %s\n", d.NewName[2:], int(hunk.NewStartLine)+i, line[1:])
+						foundForbidden = true
+					}
+				}
+			}
+		}
+	}
+	if foundForbidden {
+		return errors.New("found forbidden string")
+	}
+	return nil
+}