We should never run the pull with the rebase strategy. Ever.

That messes up with our commits. Specially with the top
one where dim expects a merge commit and it amends from and
link info.

So, let's forbid the rebase strategy overriding any user
config.

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Zhi Wang <zhi.a.wang@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>