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Skip redundant hash slides for non-chain-walking strategies

deflate_quick reads only the head of each hash chain and never walks
prev, while Z_HUFFMAN_ONLY and Z_RLE read neither. Sliding the whole
hash on every window move is wasted work for them. Add a slide_hash_head
functable entry that slides only the head table, so deflate_quick slides
just the head and the huffman and rle strategies skip the slide entirely.

About 8% faster on the level-1 no-CRC deflate benchmark, output unchanged.

1981 of 2117 branches covered (93.58%)

Branch coverage included in aggregate %.

6584 of 7410 relevant lines covered (88.85%)

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