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>
