Merge branch 'dont-mutate-command-more-than-once-on-implicit-invoke' of https://github.com/jonsequitur/command-line-api into dont-mutate-command-more-than-once-on-implicit-invoke

Author: jonsequitur

src/System.CommandLine/CommandExtensions.cs

@@ -42,7 +42,7 @@ public static Task<int> InvokeAsync(
 
         private static InvocationPipeline GetInvocationPipeline(Command command, string[] args)
         {
-            var parser = command.ImplicitParser ??=
+            var parser = command.ImplicitParser ??
                              new CommandLineBuilder(command)
                                  .UseDefaults()
                                  .Build();
@@ -63,4 +63,4 @@ public static ParseResult Parse(
             IReadOnlyCollection<char> delimiters = null) =>
             new Parser(command).Parse(commandLine);
     }
-}
+}