Insert the first two strings in the hash table after a flush.

Insert the first two strings in the hash table after a flush.

This allows deflate to generate the same output when continuing after
a Z_SYNC_FLUSH vs. using deflateSetDictionary() after a Z_FULL_FLUSH
or a deflateReset().  It also slightly improves compression when
flushing by providing two more strings to possibly match at the start
of the new block.