Do not store subfile signatures for very short blocks.

Don't bother to store subfile signatures for very short files, since it is
probably not worth the effort: we're probably best off just storing a new
copy of the data if it changes anyway.

For now, we used a fixed and non-scientific threshold of 16 kB as the
minimum size before we'll save subfile signatures.  However, short blocks
that are created to store any new chunks needed for a subfile incremental
are always indexed (since we are likely to want to use the same chunks
again in the next backup).