Fix project imports not working with serialized data

The change in e404c311 broke models that
relied on single-type inheritance:

```
CommitStatus.new(type: 'Ci::Build').class
=> Ci::Build

CommitStatus.new.tap{|c| c.assign_attributes(type: 'Ci::Build')}.class
=> CommitStatus
```

As a result, any serialized options would not be properly saved to the
database, leading to errors expecting a `Hash` vs. a `String`, for
example.

This affected all pipelines that used shared runners because the build
would error out due to this serialization error.

Closes https://gitlab.com/gitlab-org/gitlab/issues/34860

changelogs/unreleased/sh-fix-project-import-commit-status.yml

+---
+title: Fix project imports not working with serialized data
+merge_request: 19124
+author:
+type: fixed

lib/gitlab/import_export/relation_factory.rb

@@ -292,9 +292,11 @@ module Gitlab
 
             existing_object
           else
-            object = relation_class.new
-
-            # Use #assign_attributes here to call object custom setters
+            # Because of single-type inheritance, we need to be careful to use the `type` field
+            # See https://gitlab.com/gitlab-org/gitlab/issues/34860#note_235321497
+            inheritance_column = relation_class.try(:inheritance_column)
+            inheritance_attributes = parsed_relation_hash.slice(inheritance_column)
+            object = relation_class.new(inheritance_attributes)
             object.assign_attributes(parsed_relation_hash)
             object
           end

spec/fixtures/lib/gitlab/import_export/project.json

@@ -6226,7 +6226,9 @@
               "job_id": null,
               "name": "test build 1",
               "deploy": false,
-              "options": null,
+              "options": {
+                "image": "busybox:latest"
+              },
               "allow_failure": false,
               "stage": "test",
               "trigger_request_id": null,

spec/lib/gitlab/import_export/project_tree_restorer_spec.rb

@@ -283,6 +283,10 @@ describe Gitlab::ImportExport::ProjectTreeRestorer do
         it 'correctly restores association between a pipeline and a job' do
           expect(CommitStatus.all).to all(have_attributes(pipeline_id: a_value > 0))
         end
+
+        it 'restores a Hash for CommitStatus options' do
+          expect(CommitStatus.all.map(&:options).compact).to all(be_a(Hash))
+        end
       end
     end
   end