Bug with .gitignore and branch switching

Here's an easy to reproduce bug. It's the only example I know of where git legitimately loses data in a way that is unrecoverable, unexpected, and without warning.

``` git --version # git version 2.12.0

mkdir git-demo cd git-demo

git init

# Commit a file that will end up in .gitignore echo 'original settings' > mine.conf git add mine.conf git commit -m "Unknowingly committed my settings."

echo '*.conf' > .gitignore git add .gitignore git commit -m "Users shouldn't commit their settings"

# Spin off a feature branch here (but don't check it out) git branch feature

# Realize that we don't want that file committed git rm mine.conf git commit -m "Delete mine.conf"

echo 'Lots of laboriously tuned settings' > mine.conf

# Hop on the feature branch to do some work git checkout feature

# Hmmm... My settings are gone cat mine.conf # original settings

# Lemme hop back git checkout master

# Wait... they are gone for good! cat mine.conf # cat: mine.conf: No such file or directory ```