Stop Considering renamed-only Files as Additions

[alessio-signorini]

The current implementation of Git::Repository does not handle correctly file renames. These are the considered additions and create numerous issues when trying to submit Github PRs. A few of the linked issues I could find are:

1. Pronto doesn't handle file renames well (Pronto doesn't handle file renames well #69)
2. 422 - Validation Failed (422 - Validation Failed #149)
3. pronto having issues with renamed files (pronto having issues with renamed files #218)
4. Issue with renamed files and github_pull_request_review_formatter (Issue with renamed files and github_pull_request_review_formatter #259)
5. Pronto is failing to post reviews to github pull request (Pronto is failing to post reviews to github pull request #284)