When in edit-mode, don't perform full COW updates.
This resolves a bottleneck where every update while transforming
copied the entire mesh data-block, which isn't needed as the edit-mesh
is the source of the data being edited.
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- These changes where needed for this patch to function properly.
- {rBaa6f0f3d1fddb276391f384d0a69e509c927341f}
- {rB2b640622ff28977840ede56ff21cb6993b662927}
- Some tests in {T88021} found this operation took about 40% of the overall time taken to update the mesh while transforming.