Support `golangci-lint fmt --diff`

### Welcome - [x] Yes, I understand that the GitHub action repository is not the repository of golangci-lint itself. - [x] Yes, I've searched similar issues on GitHub and didn't find any. ### Your feature request related to a problem? Please describe. Support `golangci-lint fmt --diff` In https://github.com/golangci/golangci-lint-action/issues/1245, the claim is that `golangci-lint run` will run the same formatters that `golangci-lint fmt` would but I don't think that is true based on my experience. Testing strategy: 1. (using the default/standard config) 1. First auto-fixing my codebase with `go run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.6.0 run ./... --fix` 1. `go run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.6.0 run ./...` now passes 1. But `go run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.6.0 fmt ./...` changes even more things. (`--diff` also shows the changes) ### Describe the solution you'd like. Add an option to check that there are no additional changes from auto-formatters (`golangci-lint fmt --diff`) ### Describe alternatives you've considered. ```yaml jobs: lint-go: name: Lint Go runs-on: ubuntu-latest steps: - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2 - name: Setup go uses: actions/setup-go@be3c94b385c4f180051c996d336f57a34c397495 # v3.6.1 with: go-version: "stable" - name: Run golangci-lint (lint) run: go run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.6.1 run ./... --max-issues-per-linter=0 - name: Run golangci-lint (format) run: go run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.6.1 fmt ./... --diff ``` ### Additional context. _No response_