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.

I encountered the same issue. It is indeed a serious bug because you can not do anything with git repositories using this pattern negation.

Would it be difficult to solve this problem?

@Pierre Augier asks:

Would it be difficult to solve this problem?

when it was suggested, it was claimed to be difficult, leaning toward very hard:

https://www.mercurial-scm.org/pipermail/mercurial-devel/2008-March/005220.html

although perhaps now, the conventions selected by git could help resolve the ambiguities and performance issues that were outlined in 2008.

An optional prefix "!" which negates the pattern; any matching file excluded by a previous pattern will become included again. It is not possible to re-include a file if a parent directory of that file is excluded. Git doesn’t list excluded directories for performance reasons, so any patterns on contained files have no effect, no matter where they are defined.

• https://git-scm.com/docs/gitignore#_pattern_format