fix: say so when only part of the pull request could be listed

A local range shorter than the pull request, with no API to fill it in,
is still checked -- the commits the clone has are better than HEAD
alone -- but no longer in silence: the run is annotated with how many
of the commits were listed and that the others were not checked.

Author: shenxianpeng

main.py

@@ -531,7 +531,16 @@ def get_pr_commit_messages() -> list[Commit]:
         total = get_pr_event().get("commits")
         if messages and (not total or len(messages) == total):
             return messages
-        return get_messages_from_api() or messages
+        from_api = get_messages_from_api()
+        if from_api:
+            return from_api
+        if messages:
+            warn_shallow_checkout(
+                f"Only {len(messages)} of the pull request's {total} commits "
+                "could be listed",
+                "the others were not checked",
+            )
+        return messages
     except Exception as e:
         print(
             f"::warning::Failed to retrieve PR commit messages: {e}",

main_test.py

@@ -514,10 +514,12 @@ def test_a_range_cut_short_by_a_shallow_clone_asks_the_api(self):
             patch("main.get_messages_from_event_range", return_value=[("b2", "fix")]),
             patch("main.get_pr_event", return_value={"commits": 2}),
             patch("main.get_messages_from_api", return_value=[]) as api,
+            patch("main.warn_shallow_checkout") as warn,
         ):
-            # The API cannot help either: keep what the clone had.
+            # The API cannot help either: check what the clone had, and say so.
             self.assertEqual(main.get_pr_commit_messages(), [("b2", "fix")])
         api.assert_called_once()
+        self.assertIn("Only 1 of the pull request's 2 commits", warn.call_args[0][0])
 
     def test_asks_the_api_when_the_clone_holds_none_of_them(self):
         with (