git add -p

http://johnkary.net/blog/git-add-p-the-most-powerful-git-feature-youre-not-using-yet/

If you use git, you've used the command git add. But do you know about git add's "patch mode" using git add -p ?

Patch mode allows you to stage parts of a changed file, instead of the entire file. This allows you to make concise, well-crafted commits that make for an easier to read history.