Comment 7 for bug 1619362

I knew about potential cross-fs copies, but didn't realize the effect of de-sparsifying the file. IIRC we used some other mechanism in an earlier version of the code that adjusted for cross-fs copies, but I don't remember the details now.

At the very least we should warn or verify the sparseness both before the copy and afterward, or warn if we know we're doing cross-fs copies. I'm not sure of the best way to check for sparseness in Python, other that perhaps shelling-out. We'll also need an alternative for cross-fs copies in that case.