#778: Suppress builtin -h,--help from usage text when command already contains an option using -h

[TiagoMaiaL]

This PR fixes part of the behavior described in #778, fixing the generated error and usage texts to not list the builtin -h, --help:

I would expect -h to only be added if no existing flag or option already has that name.

I still need to address the other behaviors there, and to fix command invocation, applying the custom command and not the builtin help one.

Before

Error: Missing expected argument '<phrase>'

USAGE: repeat [--count <count>] [--include-counter] <phrase> --horse <horse>

ARGUMENTS:
  <phrase>                The phrase to repeat.

OPTIONS:
  --count <count>         How many times to repeat 'phrase'.
  --include-counter       Include a counter with each repetition.
  -h, --horse <horse>     The name of the talking horse.
  -h, --help              Show help information.

After

Error: Missing expected argument '<phrase>'

USAGE: repeat [--count <count>] [--include-counter] <phrase> --horse <horse>

ARGUMENTS:
  <phrase>                The phrase to repeat.

OPTIONS:
  --count <count>         How many times to repeat 'phrase'.
  --include-counter       Include a counter with each repetition.
  -h, --horse <horse>     The name of the talking horse.

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