Webhook payload contains incorrect assignee changes when MR is updated
### Summary
Updating an already assigned MR results in a webhook payload with assignees changes.
### Steps to reproduce
1. For a project, create a webhook integration for `Merge Request Events`.
2. Create an MR and assign it to a user.
3. The webhook payload will correctly have `changes.assignees.previous` as an empty array, and `changes.assignees.current` as the assigned user.
4. Push a commit to the MR branch.
5. The webhook payload again shows `changes.assignees.previous` and `changes.assignees.current`, as if it was updated.
### What is the current *bug* behavior?
Create an MR and assign it to a user:
```
{
"object_kind": "merge_request",
"event_type": "merge_request",
"object_attributes": {
"state": "opened",
"action": "open"
"changes": {
"assignees": {
"previous": [],
"current": [
{
"name": "Rene V",
"username": "rv",
"avatar_url": "https://secure.gravatar.com/avatar/"
}
]
},
```
Push a commit to the MR branch
```
{
"object_kind": "merge_request",
"event_type": "merge_request",
"object_attributes": {
"state": "opened",
"action": "update"
"changes": {
"assignees": {
"previous": [],
"current": [
{
"name": "Rene V",
"username": "rv",
"avatar_url": "https://secure.gravatar.com/avatar/"
}
]
},
```
### What is the expected *correct* behavior?
When an existing and assigned MR is updated, the webhook payload should not contain `changes.assignees` as that suggests the assignees were changed.
#### Results of GitLab environment info
Tested on GitLab.com 12.6.0-pre and 12.4.3
issue