GitHub: record whether the origin is a fork

For now this information is not used downstream, but it can be useful
for specific analysis or one-shot scheduling.

swh/lister/github/lister.py

@@ -183,6 +183,7 @@ class GitHubLister(Lister[GitHubListerState, List[Dict[str, Any]]]):
                 url=repo["html_url"],
                 visit_type="git",
                 last_update=pushed_at,
+                is_fork=repo.get("fork"),
             )
 
     def commit_page(self, page: List[Dict[str, Any]]):

swh/lister/github/tests/test_lister.py

@@ -52,6 +52,7 @@ def check_origin_4321(swh_scheduler: SchedulerInterface, lister: Lister) -> None
     assert origin_4321.last_update == datetime.datetime(
         2018, 11, 8, 13, 16, 24, tzinfo=datetime.timezone.utc
     )
+    assert origin_4321.is_fork is not None
 
 
 def check_origin_5555(swh_scheduler: SchedulerInterface, lister: Lister) -> None: