So I'm trying to ignore everything except for files under the /app/code/core/Mage/ directory.
For some reason, it works on Windows (git version 2.7.0.windows), but not in my Centos machine (git version 2.8.1).

git status on Windows:
-----------------------------
$ git status
On branch master

Initial commit

Untracked files:
  (use "git add <file>..." to include in what will be committed)

        app/

nothing added to commit but untracked files present (use "git add" to track)

git status on Centos:
-----------------------------
$ git status
On branch master

Initial commit

nothing to commit (create/copy files and use "git add" to track)

my .gitignore file:
/**
!/app/code/core/Mage/**