drm/ci: pick up -external-fixes from the merge target repo

drm/ci: pick up -external-fixes from the merge target repo

In case of the merge requests it might be useful to push repo-specific
fixes which have not yet propagated to the -external-fixes branch in the
main UPSTREAM_REPO. For example, in case of drm/msm development, we are
staging fixes locally for testing, before pushing them to the drm/drm
repo. Thus, if the CI run was triggered by merge request, also pick up
the -external fixes basing on the the CI_MERGE target repo / and branch.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Acked-by: Helen Koike <helen.koike@collabora.com>
Link: https://lore.kernel.org/r/20231008132320.762542-1-dmitry.baryshkov@linaro.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>