git_branch_exists can't cope with branches containing single quotes

[djh82]

I actually came across this while tidying up test branches after fixing #760.

users-MBP:thefuck user$ git checkout -b "let's"
fatal: A branch named 'let's' already exists.
users-MBP:thefuck user$ fuck
Traceback (most recent call last):
  File "/usr/local/bin/thefuck", line 12, in <module>
    sys.exit(main())
  File "/Users/user/git_repo/thefuck/thefuck/entrypoints/main.py", line 25, in main
    fix_command(known_args)
  File "/Users/user/git_repo/thefuck/thefuck/entrypoints/fix_command.py", line 42, in fix_command
    selected_command = select_command(corrected_commands)
  File "/Users/user/git_repo/thefuck/thefuck/ui.py", line 71, in select_command
    selector = CommandSelector(corrected_commands)
  File "/Users/user/git_repo/thefuck/thefuck/ui.py", line 34, in __init__
    self._commands = [next(self._commands_gen)]
  File "/Users/user/git_repo/thefuck/thefuck/corrector.py", line 60, in organize_commands
    first_command = next(corrected_commands)
  File "/Users/user/git_repo/thefuck/thefuck/corrector.py", line 91, in <genexpr>
    for corrected in rule.get_corrected_commands(command))
  File "/Users/user/git_repo/thefuck/thefuck/types.py", line 189, in get_corrected_commands
    new_commands = self.get_new_command(command)
  File "<decorator-gen-83>", line 2, in get_new_command
  File "/Users/user/git_repo/thefuck/thefuck/specific/git.py", line 32, in git_support
    return fn(command)
  File "<decorator-gen-82>", line 2, in get_new_command
  File "/Users/user/git_repo/thefuck/thefuck/utils.py", line 136, in eager
    return list(fn(*args, **kwargs))
  File "/Users/user/git_repo/thefuck/thefuck/rules/git_branch_exists.py", line 17, in get_new_command
    r"fatal: A branch named '([^']*)' already exists.", command.output)[0]
IndexError: list index out of range