Fix diff index handling

Fix diff index handling

Inserting text at index 0 (a0 N command, to prepend N lines) resulted in
text being appended since the diff index 0 was mapped to Ruby index -1.
The solution is to turn insertions into Array#unshift rather than
Array#<<, so that they can use the same index as the diff, and only
decrementing the diff index for deletions.