make FlagExclusivity.chooseFirst work

[glessard]

Previously, using a set of flags declared with FlagExclusivity.chooseFirst would result in an error instead of choosing the first from the command line. (See #66).

Detailed Design

Added a case handling repeating arguments in the ArgumentSet.updateFlag function.
The previous behaviour had the repeating-arguments-with-.chooseFirst situation going through the default case, which didn't add the current argument to the list of used arguments, which later caused an error due to the argument being incorrectly deemed to have not been used.

Documentation Plan

No change to documentation is needed.

Test Plan

Tests added to FlagsEndToEndTests.testParsing_invert and FlagsEndToEndTests.testParsingCaseIterable_RepeatableFlags.

Source Impact

None.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

This PR fixes #66

[natecook1000]

Thanks, @glessard! 🎉