Patch D10034

Prior to this patch, attempting to modify an existing bolt would
*replace* the object's mesh data block with a new one and remove the
original. This removal would essentially delete all linked duplicates.

Now the original mesh data block is modified in place, which avoids
having to copy anything between new and old object data blocks
(materials, names, etc), and causes all linked duplicates to reflect
the changes as users would expect.