case to handle if a pattern could spill over into another data chunk

[wurgg]

more of an edge case, but we need to handle if a pattern can continue over chunk boundaries.

// This block is what allows us to detect matches over chunk boundaries.
// When we're close enough to a boundary that a pattern match could overrun, we
// copy the final bytes in the buffer to the start of the buffer, then read into
// the rest of the buffer.