Add dynamic completion for release names

This commit adds dynamic completion for the commands
   helm status <TAB>
   helm uninstall <TAB>
   helm history <TAB>
   helm test run <TAB>
   helm upgrade <TAB>
   helm get <subcommand> <TAB>
   helm rollback <TAB>

Aliases of commands are automatically taken care of, such as
  helm delete <TAB>
which is an alias of
  helm uninstall

Support for override flags in completion is included for
such dynamic completion.

The list of release names to complete is obtained by running
  helm list $(__helm_override_flags) -a -q -m 1000 -f ${filter}
where
  ${__helm_override_flags} is any user-specified flags part of
      --kubeconfig --kube-context --home --namespace -n
  ${filter} is whatever prefix the user may have already typed
      for the release name

Signed-off-by: Marc Khouzam <marc.khouzam@ville.montreal.qc.ca>

cmd/helm/root.go

@@ -29,6 +29,61 @@ import (
 	"helm.sh/helm/pkg/registry"
 )
 
+const (
+	bashCompletionFunc = `
+__helm_override_flag_list=(--kubeconfig --kube-context --home --namespace -n)
+__helm_override_flags()
+{
+    local ${__helm_override_flag_list[*]##*-} two_word_of of var
+    for w in "${words[@]}"; do
+        if [ -n "${two_word_of}" ]; then
+            eval "${two_word_of##*-}=\"${two_word_of}=\${w}\""
+            two_word_of=
+            continue
+        fi
+        for of in "${__helm_override_flag_list[@]}"; do
+            case "${w}" in
+                ${of}=*)
+                    eval "${of##*-}=\"${w}\""
+                    ;;
+                ${of})
+                    two_word_of="${of}"
+                    ;;
+            esac
+        done
+    done
+    for var in "${__helm_override_flag_list[@]##*-}"; do
+        if eval "test -n \"\$${var}\""; then
+            eval "echo \${${var}}"
+        fi
+    done
+}
+__helm_list_releases()
+{
+	__helm_debug "${FUNCNAME[0]}: c is $c words[c] is ${words[c]}"
+	local out filter
+	# Use ^ to map from the start of the release name
+	filter="^${words[c]}"
+    if out=$(helm list $(__helm_override_flags) -a -q -m 1000 -f ${filter} 2>/dev/null); then
+        COMPREPLY=( $( compgen -W "${out[*]}" -- "$cur" ) )
+    fi
+}
+__helm_custom_func()
+{
+	__helm_debug "${FUNCNAME[0]}: last_command is $last_command"
+    case ${last_command} in
+		helm_uninstall | helm_history | helm_status | helm_test_run |\
+	    helm_upgrade | helm_rollback | helm_get_*)
+            __helm_list_releases
+            return
+            ;;
+        *)
+            ;;
+    esac
+}
+`
+)
+
 var globalUsage = `The Kubernetes package manager
 
 To begin working with Helm, run the 'helm init' command:
@@ -53,11 +108,12 @@ Environment:
 
 func newRootCmd(actionConfig *action.Configuration, out io.Writer, args []string) *cobra.Command {
 	cmd := &cobra.Command{
-		Use:          "helm",
-		Short:        "The Helm package manager for Kubernetes.",
-		Long:         globalUsage,
-		SilenceUsage: true,
-		Args:         require.NoArgs,
+		Use:                    "helm",
+		Short:                  "The Helm package manager for Kubernetes.",
+		Long:                   globalUsage,
+		SilenceUsage:           true,
+		Args:                   require.NoArgs,
+		BashCompletionFunction: bashCompletionFunc,
 	}
 	flags := cmd.PersistentFlags()