Ensure help parsing is shared by all commands

Fixes #67

Author: kzu

src/VisualStudio.Tests/CommandFactoryTests.cs

@@ -29,6 +29,14 @@ public async Task when_creating_command_with_help_argument_then_throws_show_usag
             await Assert.ThrowsAsync<ShowUsageException>(async () => await commandFactory.CreateCommandAsync("test", ImmutableArray.Create("/h")));
         }
 
+        [Fact]
+        public async Task when_creating_builtin_command_with_help_argument_then_throws_show_usage()
+        {
+            var commandFactory = new CommandFactory();
+
+            await Assert.ThrowsAsync<ShowUsageException>(async () => await commandFactory.CreateCommandAsync("modify", ImmutableArray.Create("-?")));
+        }
+
         [Theory]
         [InlineData(Commands.Install, typeof(InstallCommand))]
         [InlineData(Commands.Run, typeof(RunCommand))]

src/VisualStudio/CommandFactory.cs

@@ -115,8 +115,14 @@ public Task<Command> CreateCommandAsync(string command, ImmutableArray<string> a
             // Create the descriptor
             var commandDescriptor = factory.CreateDescriptor();
 
+            // Parse out help arg up-front to avoid unnecessary extra parsing for args.
+            var help = new HelpOption();
+            var finalArgs = help.Parse(args);
+            if (help.Value)
+                throw new ShowUsageException(commandDescriptor);
+
             // Parse the arguments
-            commandDescriptor.Parse(args);
+            commandDescriptor.Parse(finalArgs);
 
             // And create the command
             return Task.FromResult(factory.CreateCommand(commandDescriptor));