The approach is lifted from cobra: `tailscale completion bash` emits a bash
script for configuring the shell's autocomplete:
. <( tailscale completion bash )
so that typing:
tailscale st<TAB>
invokes:
tailscale completion __complete -- st
RELNOTE=tailscale CLI now supports shell tab-completion
Fixes#3793
Signed-off-by: Paul Scott <paul@tailscale.com>
