fix RegisterFlagCompletionFunc concurrent map writes error (#1423)

* fix-RegisterFlagCompletionFunc-concurrent
* set to root command
* move to non-public fields

command.go

@@ -142,6 +142,9 @@ type Command struct {
 	// that we can use on every pflag set and children commands
 	globNormFunc func(f *flag.FlagSet, name string) flag.NormalizedName
 
+	//flagCompletionFunctions is map of flag completion functions.
+	flagCompletionFunctions map[*flag.Flag]func(cmd *Command, args []string, toComplete string) ([]string, ShellCompDirective)
+
 	// usageFunc is usage func defined by user.
 	usageFunc func(*Command) error
 	// usageTemplate is usage template defined by user.