Merge branch 'fix/import-export-mr-error' into 'master'

Fix import/export merge requests error

When MWBS is set and the MR exported/imported an error gets thrown. We should ignore this so it can succeed as the merge user may no longer exist in the target instance.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/24009 and https://gitlab.com/gitlab-com/support-forum/issues/1345

See merge request !8134

app/models/merge_request.rb

@@ -97,7 +97,7 @@ class MergeRequest < ActiveRecord::Base
   validates :source_branch, presence: true
   validates :target_project, presence: true
   validates :target_branch, presence: true
-  validates :merge_user, presence: true, if: :merge_when_build_succeeds?
+  validates :merge_user, presence: true, if: :merge_when_build_succeeds?, unless: :importing?
   validate :validate_branches, unless: [:allow_broken, :importing?, :closed_without_fork?]
   validate :validate_fork, unless: :closed_without_fork?
 

changelogs/unreleased/fix-import-export-mr-error.yml

@@ -0,0 +1,4 @@
+---
+title: Fix Import/Export merge requests error while importing
+merge_request: 
+author: 

lib/gitlab/import_export/relation_factory.rb

@@ -14,7 +14,7 @@ module Gitlab
                     priorities: :label_priorities,
                     label: :project_label }.freeze
 
-      USER_REFERENCES = %w[author_id assignee_id updated_by_id user_id created_by_id].freeze
+      USER_REFERENCES = %w[author_id assignee_id updated_by_id user_id created_by_id merge_user_id].freeze
 
       PROJECT_REFERENCES = %w[project_id source_project_id gl_project_id target_project_id].freeze
 

spec/lib/gitlab/import_export/project.json

@@ -2517,7 +2517,7 @@
       "merge_params": {
         "force_remove_source_branch": null
       },
-      "merge_when_build_succeeds": false,
+      "merge_when_build_succeeds": true,
       "merge_user_id": null,
       "merge_commit_sha": null,
       "deleted_at": null,