fix: match short flags when ShortPrefix and LongPrefix are identical (#26)

[terrabeckhub]

Summary

Fixes Windows-style identical / short and long prefixes so /h matches short flags (#26).

When ShortPrefix("/") and LongPrefix("/") are the same, every flag looked both short and long. The previous rule preferred the long form, so /h was parsed as a long flag named h and failed even though a short h existed.

Change

If the prefixes are identical, prefer ShortFlag when the next character is a registered short flag (covers /h and joined values like /ifile.txt); otherwise keep LongFlag for names like /help.

Test

Adds test/windows_slash_prefix.cxx.

Closes #26

[Taywee]

I'm still not sure if I like this pattern. You should be able to specify "h", "help" to make both /h and /help long flags. This approach won't work, because if you have 'h', "help" as your flags, /help will see /h as a short flag, and then look for e as a short flag (as the failing tests show).

This kind of argument matching is pretty ugly and ambiguous. I don't like having the short flag the same as the long flag; it becomes inconsistent and difficult to reason about from a user perspective. I'd prefer making / just the long prefix with multiple forms for the arguments you want, and if anything, adding an option to forbid long or short arguments (so the parser becomes long-only or short-only).

[Taywee]

Fails tests.