Improve diff lines count efficiency

Adds a rubocop rule (with autocorrect) to ensure line break after guard clauses.

This reverts commit 1d3c33b5.

This is controlled with the feature flag gitlab_git_diff_size_limit_increase.
Both of these limits were basically picked arbitrarily in the first place;
disabling the feature flag reverts to the old limits.

This reverts commit cb10b725c8929b8b4460f89c9d96c773af39ba6b.