cmd/tailscale: add shell tab-completion

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>

cmd/tailscale/cli/file.go

@@ -26,6 +26,7 @@ import (
 	"github.com/peterbourgon/ff/v3/ffcli"
 	"golang.org/x/time/rate"
 	"tailscale.com/client/tailscale/apitype"
+	"tailscale.com/cmd/tailscale/cli/ffcomplete"
 	"tailscale.com/envknob"
 	"tailscale.com/net/tsaddr"
 	"tailscale.com/syncs"
@@ -418,6 +419,7 @@ var fileGetCmd = &ffcli.Command{
 	skip:       skip conflicting files: leave them in the taildrop inbox and print an error. get any non-conflicting files
 	overwrite:  overwrite existing file
 	rename:     write to a new number-suffixed filename`)
+		ffcomplete.Flag(fs, "conflict", ffcomplete.Fixed("skip", "overwrite", "rename"))
 		return fs
 	})(),
 }