fix: handled the case of help and list flags combination to show list

[benodiwal]

What does this PR try to resolve?

Fixes #15648

When running cargo --help, users are told to "See all commands with --list". A natural interpretation is that cargo --help --list should show the full list of commands. However, clap's automatic --help handling exits early, so --list was being ignored.

This PR makes --help --list (or -h --list, or --list --help) behave as --list, showing the full list of installed commands.

How to test and review this PR?

Before this change:

 cargo --help --list
 # Shows only the help output, --list is ignored

After this change:

cargo --help --list
Installed Commands:
    add                  Add dependencies to a Cargo.toml manifest file
    bench                Execute all benchmarks of a local package
    build                Compile a local package and all of its dependencies
    ...

[rustbot]

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[epage]

Note that the issue hasn't been accepted by the Cargo team.