Respect negated .gitignore patterns

[bagrat]

Show nodes that match negated patterns in .gitignore. For example:

!this_file_must_be_shown_in_nerdtree

[albfan]

Hey @n9code, After working with:

https://github.com/mhinz/vim-signify

and see its .gitignore

https://github.com/mhinz/vim-signify/blob/master/.gitignore#L1

I decide to reopen issue, with a failed test included.

[albfan]

I guess this is working correctly check if this fullfill your use cases

https://github.com/albfan/nerdtree-git-plugin/blob/master/t/test6.sh

[nkakouros]

This seems to work fine. I think this issue can be closed.