Comment 2 for bug 109114

I think we all agree we should handle these situations better.

If you have some python skills I'd be delighted to guide you through
addressing this error.

Basically I think the right approach is to:
 * try the fast path
 * catch MemoryError
 * fallback to a slower file-based approach

This will work for most cases, and will address the number of copies
problem substantially, but we may still fall down on merge, which is
somewhat trickier to reduce memory usage on.