Including the comment since that commit will be GC'd at some point:

> Now it all comes back... without this the [Mono build fails](https://travis-ci.org/libgit2/libgit2sharp/builds/5800298):

> ```
LibGit2Sharp.Tests.SubmoduleFixture.CanRetrieveTheCommitIdsOfASubmodule(name: "sm_gitmodules_only", headId: null, indexId: null, workDirId: null): System.ArgumentNullException : Argument cannot be null.
Parameter name: src
  at (wrapper managed-to-native) System.Runtime.InteropServices.Marshal:PtrToStructure (intptr,System.Type)
  at LibGit2Sharp.Core.Handles.OidSafeHandle.MarshalAsGitOid ()
  at LibGit2Sharp.Core.Handles.OidSafeHandle.MarshalAsObjectId ()
  at LibGit2Sharp.Core.Proxy.git_submodule_head_id (LibGit2Sharp.Core.Handles.SubmoduleSafeHandle submodule)
  at LibGit2Sharp.Submodule.get_HeadCommitId ()
  at LibGit2Sharp.Tests.SubmoduleFixture.CanRetrieveTheCommitIdsOfASubmodule (System.String name, System.String headId, System.String indexId, System.String workDirId)
```