dim: enforce consistent conflictstyle in drm-tip

Otherwise the tree rebuilding fails. This might be the reason why some
git versions create different conflicts, since checking
git-sources/rerere.c didn't provide any hint at all about what has
changed.

Note this is only --local, i.e. for the current git repo, not --global
or --system. But with worktrees that includes a lot more than just the
drm-tip repo. But then we just force the default, so hopefully the
impact is minimal.

Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
Cc: Noralf Trønnes <noralf@tronnes.org>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>