Fix wrong display of recently pushed notification (#25812) (#31043)

Backport #25812

~~ps: removed some new codes in `tests/integration/pull_merge_test.go`~~

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Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

models/fixtures/repository.yml

@@ -327,7 +327,7 @@
   is_archived: false
   is_mirror: false
   status: 0
-  is_fork: false
+  is_fork: true
   fork_id: 10
   is_template: false
   template_id: 0