Remove unwanted files from a git repo AFTER adding a .gitignore. The files will remain on disk.

git-ignore.sh

## I just ran into this after initializing a Visual Studio project _before_ adding a .gitignore file (like an idiot).  
## I felt real dumb commiting a bunch of files I didn't need to, so the commands below should do the trick.  The first two commands
## came from the second answer on this post:  http://stackoverflow.com/questions/7527982/applying-gitignore-to-committed-files

# See the unwanted files:
git ls-files -ci --exclude-standard

# Remove the unwanted files: 
git ls-files -ci --exclude-standard -z | xargs -0 git rm --cached

# Commit changes
git commit -am "Removed unwanted files marked in .gitignore"

# Push
git push origin master # or whatever branch you're on

This can also be done via the web interface.
You have to open the file, then click the ... at the top right for the delete option.