Add support for .gitignore pattern negation

See for reference: https://git-scm.com/docs/gitignore

If a pattern starts with !, it negates that pattern.

The repo clones just fine, but when running status, this error message comes up:

.gitignore: unsupported ignore pattern '!.gitignore'

Here's a sample repo that uses these types of ignore files:

git://uclibc.org/uClibc.git

that is really annying.