What type of PR is this?

• optimization

What this PR does / why we need it:

Three optimizations:

1. Previously, completions checked against all levels of subcommands. In case a cli has hundreds of subcommands, this lookup would be a significant effort. This change has it look only for top-level commands, which must necessarily be present before their children.
2. Add hidden top-level commands to the list that were previously not in the list. Before, this would confuse completions inside hidden commands. (Additional issue related to completion inside
3. Stop adding help commands in the fish completion code; ensureHelp() has already seen to this.

Which issue(s) this PR fixes:

None. I can make one if it helps. This is in preparation for #2120.

Special notes for your reviewer:

For full disclosure, I probably do not know enough about urfave/cli to say conclusively that there is no way you can "skip" a level such that a deeper sub-command could show up without being preceded by its parent?

Testing

Current main (53ad542) will give this result when more-awesome has Hidden: true before 2. above is addressed.

$ mycli more-awesome -- <tab>
--global-opt  --help  (show help)  --version  (print the version)

Release Notes

Remove superfluous parts from fish completions.

For less confusion, I'll save the bits about hidden command completion for the next PR.