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

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>
(cherry picked from commit 52cd87ea16f05fc3cc87be9315cd2d2e6dc9850d)