Use a better name for global gitignore

Renamed .gitignore to .gitignore_global.

This is also recommended here:
https://help.github.com/articles/ignoring-files#create-a-global-gitignore

What I see as benefits:
- You could have a separate `.gitignore` file in the dotfiles repository.
- `.gitignore` in the home folder is confusing when it is not actually a Git
repository.