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

Thanks @jeffjohnson9046. And also to @OstenTV for the Windows version

For Windows, if anyone needs it :)

git ls-files -ci --exclude-standard

REM Remove the unwanted files: 
FOR /F "tokens=*" %G IN ('git ls-files -ci --exclude-standard') DO git rm --cached "%G"

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

REM Push
git push origin master

For the second command a powershell verion would be:

git ls-files -ci --exclude-standard |
ForEach-Object { git rm --cached $_ }