[Proposal](https://github.com/libgit2/libgit2sharp/pull/552/files#diff-aaf0799e8d9b97c726d4fd5c616b78daR345): move `RetrieveCommitOids` to `RepositoryExtensions`. It needs a different name, but I think this works. The only downside is that it needs the overload of `Lookup` that takes option flags, which isn't exposed through `IRepository`.