[BUG] rebase --interactive silently overwrites ignored files

[wh <microrffr@gmail.com>]

I'm using git 2.20.1 from Debian. Git is usually careful not to
overwrite untracked files, including ignored files. But interactive
rebase doesn't detect this (non-interactive rebase works fine).

Reproduction:
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#!/bin/sh
mkdir upstream
cd upstream
git init
echo 1 >feature-1
git add feature-1
git commit -m "feature 1"

cd ..
git clone upstream local
cd local
# write some tools for our own convenience
echo ours >tools
echo /tools >>.git/info/exclude
# start working on a feature
git checkout -b f2
echo wip >feature-2
git add feature-2
git commit -m "wip"

cd ../upstream
# official tools are available
echo theirs >tools
git add tools
git commit -m "tools"

cd ../local
git fetch ../upstream master

# this would be okay
#git rebase FETCH_HEAD

# problem: overwrites tools silently
GIT_EDITOR=true git rebase -i FETCH_HEAD

cat tools

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Expected: `git rebase -i` fails because it would have to overwrite the
untracked "tools" file. Contents of tools file remains `ours`.

Actual: Contents of tools file becomes `theirs`.