Introduce Gitlab::Diff::Extractor
What does this MR do and why?
This class abstracts the logic for extracting diffs from comparisons (e.g. MR diffs, branch comparison) as it's being duplicated before.
MR acceptance checklist
This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.
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I have evaluated the MR acceptance checklist for this MR.
Related to #412791