Fix duplicate options showing up in command help output

[jonathanpenn]

When using option groups to bring options into subcommands that are also
used in parent commands, the help system duplicates the options in the
generated output.

For instance, in the SubcommandEndToEndTest example, the --name
argument is listed twice for both the subcommands CommandA and
CommandB, and expected by the test assertion.

It is annoying in a simple case like this but causes a lot of unhelpful
noise in help output when you share more and more options across a
command heirarchy.

To solve this we need to keep track of what
HelpGenerator.Section.Element values have already been processed from
parent commands in the command stack. I acheived this by making
Element conform to Hashable to track in a Set.

This assumes that we don't need to support re-using command names up and
down a command heirarchy. It doesn't work at all today (you get an error
when trying to use the exact same option in a parent and child
command). If we choose to support this eventually then we'll need to
augment this solution to keep track of where the Element was generated
in the heirarchy.

This also updates the test to properly assert that the --name option
is only output once for the subcommands in SubcommandEndToEndTest.

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

[natecook1000]

This looks great, @jonathanpenn — how great that we already had a test case covering the incorrect behavior. 🙃

Eventually we want to add some command definition validation that would catch the duplicated option/flag names that you mentioned. We don't need to worry about that at this point.