Fix potential OOM/panic in Apply#75
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The apply logic trusted the line counts of the incoming patch when allocating memory for preimages. This meant a patch could specify a very large old position in a fragment and force the library to allocate excessive memory before checking if the source actually contains that many lines. To fix this while keeping the current structure that reads the source incrementally, read the preimage in chunks of 4096 lines. This bounds the memory that might be allocated before we reach the end of the source file. Most practical patches should only require one or two reads. Also fix a related panic that could happen with an old position close to math.MaxInt64.
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The apply logic trusted the line counts of the incoming patch when allocating memory for preimages. This meant a patch could specify a very large old position in a fragment and force the library to allocate excessive memory before checking if the source actually contains that many lines.
To fix this while keeping the current structure that reads the source incrementally, read the preimage in chunks of 4096 lines. This bounds the memory that might be allocated before we reach the end of the source file. Most practical patches should only require one or two reads.
Also fix a related panic that could happen with an old position close to
math.MaxInt64.