From http://stackoverflow.com/questions/1139762/ignore-files-that-have-already-been-committed-to-a-git-repository#answer-1139797

re-gitignore.sh

# To untrack a single file that has already been added/initialized to your 
# repository, i.e., stop tracking the file but not delete it from your system use: 
  
  git rm --cached filename

# To untrack every file that is now in your .gitignore:
# First commit any outstanding code changes, and then, run this command:

  git rm -r --cached .

# This removes any changed files from the index(staging area), then just run:

  git add .

# Commit it:

  git commit -m ".gitignore is now working"

#To undo `git rm --cached filename`, use `git add filename`.