Fix for #29676, MR webhooks will now show changes in assignees.

ee/changelogs/unreleased/29676-mr-webhooks-missing-assignee-changes.yml

+---
+title: Restores data for assignee changes in merge request webhooks.
+merge_request: 16812
+author: Jesse Hall @jessehall3
+type: fixed

lib/gitlab/hook_data/merge_request_builder.rb

@@ -34,6 +34,7 @@ module Gitlab
       end
 
       SAFE_HOOK_RELATIONS = %i[
+        assignees
         labels
         total_time_spent
       ].freeze

spec/lib/gitlab/hook_data/issuable_builder_spec.rb

@@ -42,7 +42,15 @@ describe Gitlab::HookData::IssuableBuilder do
               [{ id: 1, title: 'foo' }],
               [{ id: 1, title: 'foo' }, { id: 2, title: 'bar' }]
             ],
-            total_time_spent: [1, 2]
+            total_time_spent: [1, 2],
+            assignees: [
+              [],
+              [{
+                name: "Foo Bar",
+                username: "foobar",
+                avatar_url: "http://www.example.com/my-avatar.jpg"
+              }]
+            ]
           }
         end
         let(:data) { builder.build(user: user, changes: changes) }
@@ -58,6 +66,14 @@ describe Gitlab::HookData::IssuableBuilder do
             total_time_spent: {
               previous: 1,
               current: 2
+            },
+            assignees: {
+              previous: [],
+                current: [{
+                  name: "Foo Bar",
+                  username: "foobar",
+                  avatar_url: "http://www.example.com/my-avatar.jpg"
+                }]
             }
           }))
         end