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/launcher.sh

@@ -0,0 +1,47 @@
+#!/usr/bin/env bash
+# ts-git-hook launcher (installed at .git/hooks/ts-git-hook).
+#
+# Written by misc/install-git-hooks.go from the canonical copy embedded
+# in tailscale.com/misc/git_hook/githook. On every invocation it:
+#
+#   1. Compares misc/git_hook/HOOK_VERSION against the binary's version.
+#   2. If stale or missing: rebuilds .git/hooks/ts-git-hook-bin and runs
+#      `ts-git-hook-bin install` to refresh the launcher and per-hook
+#      wrappers.
+#   3. Execs the binary with the hook's args.
+#
+# Any change to this file or the binary must bump HOOK_VERSION.
+set -euo pipefail
+
+REPO_ROOT="$(git rev-parse --show-toplevel 2>/dev/null)" || {
+  echo "git-hook: not in a git repo" >&2
+  exit 1
+}
+
+HOOK_DIR="$(git -C "$REPO_ROOT" rev-parse --git-path hooks)"
+case "$HOOK_DIR" in
+/*) ;;
+*) HOOK_DIR="$REPO_ROOT/$HOOK_DIR" ;;
+esac
+
+# Windows (Git for Windows / MSYS2) needs .exe suffixes.
+EXE=""
+case "$(uname -s)" in MINGW* | MSYS* | CYGWIN*) EXE=".exe" ;; esac
+
+BINARY="$HOOK_DIR/ts-git-hook-bin$EXE"
+WANT="$(cat "$REPO_ROOT/misc/git_hook/HOOK_VERSION" 2>/dev/null || echo 0)"
+HAVE="$("$BINARY" version 2>/dev/null || echo none)"
+
+if [ "$WANT" != "$HAVE" ]; then
+  GO="$REPO_ROOT/tool/go$EXE"
+  if [ ! -x "$GO" ]; then GO=go; fi
+  echo "git-hook: rebuilding ts-git-hook-bin..." >&2
+  (cd "$REPO_ROOT" && "$GO" build -o "$BINARY" ./misc/git_hook) || {
+    echo "git-hook: rebuild failed, run: ./tool/go run ./misc/install-git-hooks.go" >&2
+    exit 1
+  }
+  "$BINARY" install
+fi
+
+exec "$BINARY" "$@"
+