Add generated_files to Gitlab::Git::Compare

What does this MR do and why?

Change filter_generated_files as we actually just want to list generated files instead. Also, it doesn't need to be at Repository model so move it to the raw repository.

We also want to be able to detect generated files automatically, but we need to wait for a new gitaly RPC for that so just handle manual overrides for now.

MR acceptance checklist

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Related to #391776 (closed)