Repeated flags with FlagExclusivity.chooseFirst error instead of choosing first.

[glessard]

Repeated flags defined with FlagExclusivity.chooseFirst error instead of choosing first.

ArgumentParser version: master at 58660b0
Swift version: Apple Swift version 5.2 (swiftlang-1103.0.30 clang-1103.0.30)

Checklist

• If possible, I've reproduced the issue using the master branch of this package
• I've searched for existing GitHub issues

Steps to Reproduce

Use this simple program:

enum Enum: String, CaseIterable { case s, t, u }

struct Options: ParsableCommand {
  @Flag(name: .short, exclusivity: .chooseFirst)
  var flag1: Enum
}

let options = Options.parseOrExit()
print(options.flag1)

$ options -s -t

Expected behavior

It should print "s" by itself, on a line

Actual behavior

$ options -s -t
Error: Unknown option '-t'
Usage: options -s -t -u