Accessing previous state of resource from gitlab webhooks - api

Question

In implementing a custom webhook handler for Gitlab issues, I'm struggling to find out how to determine the previous value of a field before the hook fired.
For reference, this is the webhooks documention, and an issue-specific payload looks a bit like:
{
"object_attributes": {
"action": "update",
"url": "https://example.com/foo/bar/issues/10",
"iid": 10,
"state": "opened",
"milestone_id": null,
"description": "44",
"branch_name": null,
"position": 0,
"updated_at": "2015-11-07 17:39:47 +0000",
"created_at": "2015-11-07 17:06:07 +0000",
"project_id": 51,
"author_id": 6,
"assignee_id": null,
"title": "123",
"id": 180
},
"user": {
"avatar_url": "https://example.com/uploads/user/avatar/6.jpg",
"username": "shabble",
"name": "real name"
},
"object_kind": "issue"
}
when an issue is modified.
In this example, the assignee_id was changed from myself to null, but there appears to be no way to determine what the previous value (that is, me) was prior to update.
This is the same for almost all editable fields, including things like the entire issue description. So it's possible to tell what it is now, and maybe when it was last changed, but not see any of the previous changes.
Is there anywhere this information (previous state) can be found, perhaps via the gitlab issues API although I've not found anything plausible.
The only option seems to be a separate system to record the values from every hook, and then when a subsequent hook comes in that matches the same issue, diff the last record to determine changes. This would be less than ideal.
Any other ideas?