Do not store subfile signatures for very short blocks.

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).