[Fix #6262] Optimise --auto-gen-config when Metrics/LineLength cop is disabled

[tom-lord]

Resolves #6262

With this change, running rubocop --auto-gen-config will now skip "Phase 1 of 2: run Metrics/LineLength cop", if the Metrics/LineLength cop is disabled.

This makes the todo file generation faster, when line length checks are disabled.

[jonas054]

@tom-lord Looks good, but please follow your own suggestion and present a different message when the cop is disabled.

[tom-lord]

[BUMP]

I have made further changes, as discussed above with @jonas054.
"Phase 1" of --auto-gen-config (Metrics/LineLength cop) will now be skipped if overridden or disabled, with an accurate corresponding message.

[jonas054]

Please update CHANGELOG.md.

[jonas054]

You need to add a link to your GitHub account at the bottom of the file.

[tom-lord]

Ahh right, yeah.

I figured it was just some CircleCI glitch since my final commit would "obviously" not break any tests. The error message is quite clear, but unfortunately the output is so long that you need to download the full log to even see it!

I might submit a separate PR to flatten that output into something legible, for the next person.

[jonas054]

👍 Looks good now!

[tom-lord]

Is there anything holding this up from being merged? I've just had to rebase again, as the changelog diff became stale.

[jonas054]

I resolved the conflict and merged the PR. Thanks, @tom-lord!
Cc @bbatsov