Support for ignore patterns that negate other patterns

[sirnewton01]

The '!' operator is meant to negate other ignore patterns. This task will involve detecting that operator and apply the precedence rules for the negation of other patterns.

When complete, try not skipping the t0008-ignores test cases. It is expected that most of them should be passing once the negation is implemented.

[driusan]

I'm working on implementing sparse checkout, and this is also breaking the tests for that. (It's difficult to tell if it's the only problem or not because the error happens relatively early and the tests run consecutively.)