copy: process empty extents more efficiently

copy: process empty extents more efficiently

* src/copy.c (extent_copy): Treat an allocated but empty extent
much like a hole.  I.E. don't read data we know is going to be NUL.
Also we convert the empty extent to a hole only when SPARSE_ALWAYS
so that the source and dest have the same allocation.  This will
be improved soon, when we use fallocate() to do the allocation.
* tests/cp/fiemap-empty: A new test for efficiency and correctness
of copying empty extents.
* tests/Makefile.am: Reference the new test.
* NEWS: Mention the change in behavior.