fix(vulnfeeds): deduplicate commit hashes on AffectedCommits

[jess-lowe]

fixes #3523

• added deduplication logic when duplicate commit hashes are found.
• Added a testcase to test VersionInfo end-to-end to ensure no duplicate hashes.
• Also added a series of unit tests to test deduplication logic

There's probably a less convoluted way to do it, so would appreciate input.