Bug with .gitignore and branch switching

[Nevada Sanchez <sanchez.nevada@gmail.com>]

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
```