Fix merge requests not importing for older projects

This restores the merge request commit user fields in import_export.yml
for the import case.

https://gitlab.com/gitlab-org/gitlab/-/merge_requests/73307 dropped
these YAML fields because supposedly the import would fail if the
database columns didn't exist, but I haven't found that to be the
case. I tested with an import with:

1. Project generated in v13.12 that had these columns.
2. Project generated with the latest nightly build.

The import works in either case because we call
`MergeRequestDiffCommit.new` with the sanitized attributes in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/73307.

Based on the discussion in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/72219, these
attributes are needed for **import** but excluded for **export**. I
think Yorick might have assumed that since the code in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/73307 manually
removed these fields, we wouldn't need them anymore. However, without
these attributes, imports were failing to import with an older GitLab
version (e.g. v13.12) because the attributes were sanitized before the
importer got a chance to use them.

Changelog: fixed

lib/gitlab/import_export/project/import_export.yml

@@ -393,6 +393,10 @@ included_attributes:
     - :sorted
     - :diff_type
   merge_request_diff_commits:
+    - :author_name
+    - :author_email
+    - :committer_name
+    - :committer_email
     - :relative_order
     - :sha
     - :authored_date