scope duplicate-flag detection to command boundaries

[metsw24-max]

False duplicate error across command boundaries

Duplicate detection shares one used-flag set as it walks the tree, and since a command keeps parent == nullptr it acts as its own detection root. Declaring a flag inside a command and then a same-named flag in a parent scope (say run's -m followed by a top-level -m) is rejected as a duplicate even though the two live in separate namespaces; the reverse declaration order and reuse between sibling commands are already accepted, so the behaviour is order dependent. Guarding the descent with IsGroup(), which is false for a command, keeps detection within a single scope while genuine duplicates in the parser, a group, or one command are still caught. The added tests fail on master and pass with the change.

[Taywee]

I was a bit worried about this possible edge case. I probably should have asked for this to be covered by tests in the original PR.

Thanks for the fix.