Allow enabling/disabling new-from-rev per linter

[flimzy]

Your feature request related to a problem? Please describe.

Some linters are only effective when considering code in aggregate, which makes considering only "changed lines" rather ineffective. unused is a prime example. Given the following contrived diff:

 func foo() { /* ... */ }
 
-func bar() {
-   foo()
-}

we would naturally want foo() to now be detected as unused, but with the use of new-from-rev, this is not the case.

Describe the solution you'd like.

I would like the option to disable/ignore the new-from-rev flag for particular linters. unused is the prime use case, but there are likely others where this would be useful as well.

Naturally, this would require that all historical unused code (or other offending lint) be cleaned up, but that's a much smaller chore than cleaning up all lint for the entire project.

Describe alternatives you've considered.

The only alternative I've considered is keeping two .golangci.toml files, with a different subest of linters in each. While this would work reasonably well in a CI pipeline, it does not work nearly as well for local development, particularly when needing to integrate with various editors/IDEs, where custom configuration of the on-save linter can be cumbersome.

This most likely would mean that only one of the linter configs would run locally.

Not a complete deal-breaker, but not ideal.

Additional context.

No response