.gitignore cascaded negations

[oomathias]

This is not really an issue. I'm just linking issues to make sure it's going to be fixed. Please delete this issue if it's not required.

Atom has weird behavior with cascaded ignore, like described here:
atom/tree-view#196 (comment)

I went to test with libgit2 and found some bug. Some of these are already fixed in the latest version. Some will be in the next release: libgit2/libgit2#5020, libgit2/libgit2#5040

These fixes might be able to close the 5y old tree-view issue. But, we need to ensure that Atom passes a trailing slash when checking for a directory. (I didn't check that part in the code, maybe it's already the case).

[winstliu]

Thanks for the update @oomathias! We updated to a more recent version of libgit2 recently and plan to update to the next version as per #92.