If I read the ontology correctly, there is nothing in there to actually include the diff.
You could try to extend it as in:
<https://heptapod.logilab.fr/changesets/abcd123> a dcs:Changeset ;
fedeproxy_namespace:has_text_diff "<diff in textual form>" ;
rdfs:label "ci: fixing the typo in .gitlab-ci.yml" ;
dc:creator <https://heptapod.logilab.fr/users/nico> ;
dc:date "2021-01-20:18:06"^^xsd:datetime.
I tried to answer quickly, but this probably needs more thought.