Reuse the loaded commit in CommitSignature#verification_status

What does this MR do and why?

The commits list page shows the signature status of each commit. For a verified signature, CommitSignature#verification_status checks the committer email. To do this, it loaded the commit again with project.commit(commit_sha). This made one Gitaly FindCommit call for each verified commit on the page. On projects where most commits are verified, this adds up quickly.

Gitlab::Repositories::BaseSignedCommit#signature already has the commit in memory, so it now gives that commit to the signature record. CommitSignature#commit also memoizes the commit for records that do not get one. The verification status does not change, and the FindCommit call for each commit is removed.

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