Fix unrecognized -help flag with default command (#612)

When there's a default subcommand that captures pass-through args,
only a standalone help flag should trigger help (since the flag should
otherwise go to the subcommand). This fix makes that true for single-
dash help flag names, which were previously being skipped, due to
single-dash flags being lexed as both one whole flag and as a group of
individual short flags.

Re: swiftlang/swift-package-manager#7218
Re: rdar://120422808

Author: natecook1000

Sources/ArgumentParser/Parsing/CommandParser.swift

@@ -84,7 +84,7 @@ extension CommandParser {
     _ split: SplitArguments,
     requireSoloArgument: Bool = false
   ) throws {
-    guard !requireSoloArgument || split.count == 1 else { return }
+    guard !requireSoloArgument || split.originalInput.count == 1 else { return }
 
     // Look for help flags
     guard !split.contains(anyOf: self.commandStack.getHelpNames(visibility: .default)) else {

Tests/ArgumentParserEndToEndTests/DefaultSubcommandEndToEndTests.swift

@@ -11,7 +11,7 @@
 
 import XCTest
 import ArgumentParserTestHelpers
-import ArgumentParser
+@testable import ArgumentParser
 
 final class DefaultSubcommandEndToEndTests: XCTestCase {
 }
@@ -191,3 +191,28 @@ extension DefaultSubcommandEndToEndTests {
     XCTAssertThrowsError(try MyCommand.parseAsRoot(["plugin", "--verbose", "my-plugin"]))
   }
 }
+
+extension DefaultSubcommandEndToEndTests {
+  struct RootWithPassthroughDefault: ParsableCommand {
+    static let configuration = CommandConfiguration(
+      subcommands: [PassthroughDefault.self],
+      defaultSubcommand: PassthroughDefault.self,
+      helpNames: [.short, .long, .customLong("help", withSingleDash: true)]
+    )
+  }
+  
+  struct PassthroughDefault: ParsableCommand {
+    @Argument(parsing: .captureForPassthrough)
+    var remaining: [String] = []
+  }
+
+  // Test fix for https://github.com/apple/swift-package-manager/issues/7218
+  func testHelpWithPassthroughDefault() throws {
+    AssertParseCommand(
+      RootWithPassthroughDefault.self, HelpCommand.self, ["-h"]) { _ in }
+    AssertParseCommand(
+      RootWithPassthroughDefault.self, HelpCommand.self, ["-help"]) { _ in }
+    AssertParseCommand(
+      RootWithPassthroughDefault.self, HelpCommand.self, ["--help"]) { _ in }
+  }
+}