bin/gen_release_notes.py: parse "Fixes" tags as well as "Closes"

Some commits refer to bugs being fixed with "Fixes" tag. Example of one:

    e13d53e1fdb 'Revert "glx/dri: Fix DRI drawable release at MakeCurrent time"'

Parse this tag as well.

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

@@ -164,6 +164,18 @@ async def test_gather_commits():
             ''',
             [],
         ),
+        (
+            '''\
+            Parse "Fixes:" tag too
+
+            Fixes: https://gitlab.freedesktop.org/mesa/mesa/issues/36
+            Fixes: 142565a3bc2
+            Fixes: 142565a3bc2 ("docs: do something very useful")
+            Fixes: 142565a3bc2 ("docs: fix #1234, have a comma")
+            Fixes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/37
+            ''',
+            ['36', '37'],
+        ),
     ])
 async def test_parse_issues(content: str, bugs: typing.List[str]) -> None:
     mock_com = mock.AsyncMock(return_value=(textwrap.dedent(content).encode(), ''))