add fish shell completion

fixes linkerd#4208

cli/cmd/completion.go

@@ -29,15 +29,22 @@ func newCmdCompletion() *cobra.Command {
   source <(linkerd completion zsh)
 
   # zsh on osx / oh-my-zsh
-  linkerd completion zsh > "${fpath[1]}/_linkerd"`
+  linkerd completion zsh > "${fpath[1]}/_linkerd"
+
+  # Fish:
+  $ linkerd completion fish | source
+
+  # fish  To load completions for each session, execute once:
+  $ linkerd completion fish > ~/.config/fish/completions/linkerd.fish
+`
 
 	cmd := &cobra.Command{
-		Use:       "completion [bash|zsh]",
-		Short:     "Output shell completion code for the specified shell (bash or zsh)",
-		Long:      "Output shell completion code for the specified shell (bash or zsh).",
+		Use:       "completion [bash|zsh|fish]",
+		Short:     "Output shell completion code for the specified shell (bash, zsh or fish)",
+		Long:      "Output shell completion code for the specified shell (bash, zsh or fish).",
 		Example:   example,
 		Args:      cobra.ExactArgs(1),
-		ValidArgs: []string{"bash", "zsh"},
+		ValidArgs: []string{"bash", "zsh", "fish"},
 		RunE: func(cmd *cobra.Command, args []string) error {
 			out, err := getCompletion(args[0], cmd.Parent())
 			if err != nil {
@@ -62,8 +69,11 @@ func getCompletion(sh string, parent *cobra.Command) (string, error) {
 		err = parent.GenBashCompletion(&buf)
 	case "zsh":
 		err = parent.GenZshCompletion(&buf)
+	case "fish":
+		err = parent.GenFishCompletion(&buf, true)
+
 	default:
-		err = errors.New("unsupported shell type (must be bash or zsh): " + sh)
+		err = errors.New("unsupported shell type (must be bash, zsh or fish): " + sh)
 	}
 
 	if err != nil {

go.mod

@@ -45,7 +45,7 @@ require (
 	github.com/shurcooL/httpfs v0.0.0-20190707220628-8d4bc4ba7749 // indirect
 	github.com/shurcooL/vfsgen v0.0.0-20181202132449-6a9ea43bcacd
 	github.com/sirupsen/logrus v1.6.0
-	github.com/spf13/cobra v0.0.5
+	github.com/spf13/cobra v1.0.0
 	github.com/spf13/pflag v1.0.5
 	github.com/stretchr/testify v1.5.1 // indirect
 	github.com/wercker/stern v0.0.0-20190705090245-4fa46dd6987f