Fix IsOptional method for arguments (#1862)

* Fix IsOptional method for arguments

* Add test

* Add shared arg test

* Revert "Add test"

This reverts commit 5959570.

* Update src/System.CommandLine.Tests/Help/HelpBuilderTests.cs

Co-authored-by: Jon Sequeira <[email protected]>

Author: iBicha

src/System.CommandLine.Tests/Help/HelpBuilderTests.cs

@@ -932,7 +932,38 @@ public void Command_arguments_with_default_values_that_are_enumerable_display_pi
             _console.ToString().Should().Contain(expected);
         }
 
+        [Fact]
+        public void Command_shared_arguments_with_one_or_more_arity_are_displayed_as_being_required()
+        {
+            var arg = new Argument<string[]>
+            {
+                Name = "shared-args",
+                Arity = ArgumentArity.OneOrMore
+            };
+
+            var inner = new Command("inner", "command help")
+            {
+                arg
+            };
+            _ = new Command("outer", "command help")
+            {
+                inner,
+                arg
+            };
+            _ = new Command("unused", "command help")
+            {
+                arg
+            };
 
+            _helpBuilder.Write(inner, _console);
+
+            var expected =
+                $"Usage:{NewLine}" +
+                $"{_indentation}outer <shared-args>... inner <shared-args>...";
+
+            _console.ToString().Should().Contain(expected);
+        }
+        
         #endregion Arguments
 
         #region Options

src/System.CommandLine/Help/HelpBuilder.cs

@@ -325,12 +325,8 @@ private string FormatArgumentUsage(IReadOnlyList<Argument> arguments)
             {
                 StringBuilderPool.Default.ReturnToPool(sb);
             }
-
-            bool IsMultiParented(Argument a) =>
-                a.FirstParent is not null && a.FirstParent.Next is not null;
-
+            
             bool IsOptional(Argument argument) =>
-                IsMultiParented(argument) ||
                 argument.Arity.MinimumNumberOfValues == 0;
         }