Is your feature request related to a problem? Please describe.

Scan only PR commits for Gitleaks instead of whole codebase, when VALIDATE_ALL_CODEBASE is set to false

Describe the solution you'd like

When VALIDATE_ALL_CODEBASE is set to false, and megalinter runs on PR then Gitleaks scans only PR commits.

Describe alternatives you've considered

Described scenarios is achievable by passing REPOSITORY_GITLEAKS_ARGUMENTS env contains gitleaks flag --log-opts, but nice to have out-of-the-box support (using VALIDATE_ALL_CODEBASE) without extra configuration.

- name: MegaLinter
  if: ${{ github.event_name == 'pull_request' }}
  id: megalinter
  uses: oxsecurity/megalinter/flavors/documentation@v6
  env:
    VALIDATE_ALL_CODEBASE: false
    REPOSITORY_GITLEAKS_ARGUMENTS: --log-opts '--no-merges --first-parent ${{ github.event.pull_request.base.sha }}^..${{ github.event.pull_request.head.sha }}'

Downside - to scan only PR commits with proposed gitleaks config, git checkout has to be set with fetch-depth: 0

- name: Checkout
  uses: actions/checkout@v3
  with:
    fetch-depth: 0

Additional context

N/A