Bad performance with diffChars method.

It's really slow when you have lots of content remove in new string.
``` js
const fs = require('fs')
const diff =require('diff')

let arr = new Array(3000)
let content = arr.fill('a').join('\n')
let ts = Date.now()
diff.diffChars('', content)
console.log('time:' + (Date.now() - ts) + 'ms')
console.log('length:' + content.length)
```
Result:
```
time:2985ms
length:5999
```
