fix(reviewer-roulette): include deleted files in category detection

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What does this MR do and why?

Describe in detail what your merge request does and why.

• This MR fixes Issue #83 (closed), where reviewer roulette would not run on merge requests that only contain deleted files.

Example MR which reproduced the issue - gitlab-org/gitlab!198391 (closed)

• I tested this using a personal repository with Danger Bot integrated into the pipeline.
• I modified the pipeline to pull the gem from my fork of the gitlab-dangerfiles repository, which included the fix, instead of the default source.
• Here are the results:
  • Merge request reproducing the issue: https://gitlab.com/vj35.cool/test-gitlab-dangerbot/-/merge_requests/4
  • Merge request using the fixed Dangerfile gem: https://gitlab.com/vj35.cool/test-gitlab-dangerbot/-/merge_requests/6 (You can see that reviewer roulette ran successfully)

Screenshot

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Closes #83 (closed)