Don't list file duplicates in changes

What does this MR do and why?

Related to gitlab-org/gitlab#359052 (closed)

Currently, the changes.files method sometimes returns doubled filenames. This is because this method relies on the result of this block, which adds a new Change object for each file - category pair. This further causes our CI to report some files twice [example]. I wanted to eliminate this problem at its root cause. I looked through the usage of all_changed_files method in gitlab project and it seemed like it did not depend on getting doubled filenames anywhere.

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