git cherry-pick adds more lines than expected

git-cherry-pick-test

#!/bin/bash

git init
echo 1 > file

# This is the magic line.
echo * merge=union > .gitattributes

git add .
git commit -m 1
git checkout -b test
echo 2 >> file
git commit -am 2
echo 3 >> file
git commit -am 3
git checkout -

# Without merge=union, this results in a conflict.
git cherry-pick test

The resulting graph looks like this:

  master
    ↓
1 - 3'
 \
   2 - 3
       ↑
      test