Use line comparison algorithm.

This implementation fails on the following tests:
- *indent of a weirdly indented vendor prefixed CSS*: nothing much can be done, the problem rises because there is too few correct indents/deindents in this file;
- *indentation stats for tabs*: I believe the test is incorrect, there really are 8 tabs.
Any reason why?

I took the habit to do that because it's cheaper/faster for dumb interpreters, they do not have to save the previous value.