Add diff attribute back to excluded list for Import/Export

lib/gitlab/import_export/project/import_export.yml

@@ -413,7 +413,6 @@ included_attributes:
     - :b_mode
     - :too_large
     - :binary
-    - :diff
   metrics:
     - :created_at
     - :updated_at
@@ -797,6 +796,7 @@ excluded_attributes:
     - :verification_checksum
     - :verification_failure
   merge_request_diff_files:
+    - :diff
     - :external_diff_offset
     - :external_diff_size
     - :merge_request_diff_id

lib/gitlab/import_export/project/relation_factory.rb

@@ -131,7 +131,9 @@ module Gitlab
         end
 
         def setup_diff
-          @relation_hash['diff'] = @relation_hash.delete('utf8_diff')
+          diff = @relation_hash.delete('utf8_diff')
+
+          parsed_relation_hash['diff'] = diff
         end
 
         def setup_pipeline

spec/lib/gitlab/import_export/safe_model_attributes.yml

@@ -253,7 +253,6 @@ MergeRequestDiffFile:
 - b_mode
 - too_large
 - binary
-- diff
 MergeRequestContextCommit:
 - id
 - authored_date