split: avoid unnecessary input buffering

split: avoid unnecessary input buffering

Input buffering is best avoided because it introduces
delayed processing of output for intermittent input,
especially when the output size is less than that of
the input buffer.  This is significant when output
is being further processed which could happen if split
is writing to precreated fifos, or through --filter.

If input is arriving quickly from a pipe then this will
already be buffered before we read it, so fast arriving
input shouldn't be a performance issue.

* src/split.c (lines_split, lines_bytes_split, bytes_split,
lines_chunk_split, bytes_chunk_extract): s/full_read/safe_read/.
* THANKS.in: Mention the reporter.
* NEWS: Mention the improvement.