Fix cookiecutter overwrite if exists option

[NathanUrwin]

$ cookiecutter gh:nathanurwin/cookiecutter-git --no-input -f
Removing '/home/user/Projects/.github/cookiecutter-git-demo/NOTICE'...
git init
Reinitialized existing Git repository in /home/user/Projects/.github/cookiecutter-git-demo/.git/

git status
On branch master
Your branch is up-to-date with 'origin/master'.
nothing to commit, working tree clean

git add -A
git status
On branch master
Your branch is up-to-date with 'origin/master'.
nothing to commit, working tree clean

1: ['git', 'commit', '-m', 'Initial commit']
On branch master
Your branch is up-to-date with 'origin/master'.
nothing to commit, working tree clean

Traceback (most recent call last):
  File "/tmp/tmp4emWQB.py", line 201, in <module>
    main()
  File "/tmp/tmp4emWQB.py", line 196, in main
    setup_git_repo()
  File "/tmp/tmp4emWQB.py", line 123, in setup_git_repo
    run(['git', 'commit', '-m', 'Initial commit'])
  File "/tmp/tmp4emWQB.py", line 34, in run
    raise error
subprocess.CalledProcessError: Command '['git', 'commit', '-m', 'Initial commit']' returned non-zero exit status 1
ERROR: Stopping generation because post_gen_project hook script didn't exit successfully
Hook script failed (exit status: 1)