bin/gen_release_notes.py: don't fail if "Closes" refers to an MR

Sometimes a tag "Closes:" in a commit may refer to a merge request
instead of an issue. Examples of such commits:

    34319c7d84 "ci/freedreno: disable antichambers trace"
    998122d9c2 "mesa: fix GL_INVALID_OPERATION in glEGLImageTargetTexStorageEXT"

Avoid failing on these by explicitly checking that the URL refers to an
issue

Cc: mesa-stable
Signed-off-by: Konstantin Kharlamov <Hi-Angel@yandex.ru>
Reviewed-by: Eric Engestrom <eric@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20241>

bin/gen_release_notes_test.py

@@ -148,6 +148,22 @@ async def test_gather_commits():
             ''',
             ['3456', '3457', '3458'],
         ),
+        (
+            '''\
+            Without /-/
+
+            Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/36
+            ''',
+            ['36'],
+        ),
+        (
+            '''\
+            Ignore merge_requests
+
+            Closes: https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20241
+            ''',
+            [],
+        ),
     ])
 async def test_parse_issues(content: str, bugs: typing.List[str]) -> None:
     mock_com = mock.AsyncMock(return_value=(textwrap.dedent(content).encode(), ''))