CommitService::FilterShasWithSignatures

The old RPC was "wrong" (inefficient at least). The code looks like the following, so this design makes more sense...

def shas_with_signatures(repository, shas)
  shas.select do |sha|
    begin
      Rugged::Commit.extract_signature(repository.rugged, sha)
    rescue Rugged::OdbError
      false
    end
  end
end

Closes gitaly#746 (closed)