dim: Enforce review requirements

We can't check this when applying (since r-b/a-b tags often get added
afterwards), but we can check this when pushing. This only looks at
patches authored by the pusher.

Also update the docs to highlight that review requirements hold
especially also for bugfixes.

v2: committer instead of committer (Sean/Chris).

Cc: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Cc: Lukas Wunner <lukas@wunner.de>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian König <deathsimple@vodafone.de>
Cc: Sean Paul <seanpaul@chromium.org>
Acked-by: Sean Paul <seanpaul@chromium.org>
Acked-by: Jani Nikula <jani.nikula@linux.intel.com>
Reviewed-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>