Merging brands no longer fails or leaves products on the old brand

Merging two brands whose products shared a part number showed no warning in the preview and then failed with an internal error. And after a merge went through, the merged products still belonged to the deleted brand: the list showed the surviving brand, filtering by it found nothing, and your channels got the old name. The preview now reports every conflict the merge would hit, and a finished merge updates every product it touched. Merges finished before this fix aren't revisited. Merging is described in Brands.