[Bf-codereview] Potential fix for bug #31880 (issue 6332047)

[sergey.vfx at gmail.com]

Just sone more possible issue: when you join objects synchronization of
subdivision level happens in multiresModifier_prepare_join(). This could
have the same issue if you're joining objects with different multires
levels and which have got seevral instances.

Apart from this - LGTM.

http://codereview.appspot.com/6332047/