Most projects have automatically generated files or folders from the operating system, applications, package managers etc. Usually, we don‘t want to include these types of things in our remote repos because they can clutter the git history/storage and are not applicable to everyone that works on the project. In this lesson, we show how to create a .gitignore file to ignore files and folders from being tracked by git.
If before the files or folders are added staging area, we can add those into .gitingore file, so it won‘t be tracked.
But If we also add those file or folder into the staging area, we can run:
git rm --cached
[Practical Git] Remove unnecessary git tracking with .gitignore files
原文：http://www.cnblogs.com/Answer1215/p/5763471.html