Currently, the signatures are calculated as

```
commits = commits.map do |commit|
  case commit.signature_type
    when :PGP
      Gitlab::Gpg::Commit.new(commit).signature
    when :X509
      Gitlab::X509::Commit.new(commit).signature
  end
end
```

In this case, the `lazy_signature&.itself` is directly called
for every commit, but at that point, `BatchLoader` doesn't the
information about a single commit and performs the query on every call.

If we split the code into two steps: initialization and invocation:

```
commits = commits.map do |commit|
  case commit.signature_type
    when :PGP
      Gitlab::Gpg::Commit.new(commit)
    when :X509
      Gitlab::X509::Commit.new(commit)
  end
end
```

will call `lazy_signature` for every commit, i.e store the information
for a single one so on the invocation step:

```
commits.each { |commit| commit.signature }
```

`lazy_signature&.itself` will perform a single batch query since
it already has the info about a single commit