Suggest git checkout -b (#754)

This fixes https://github.com/nvbn/thefuck/issues/632

This uses `script_parts` instead of `script.startswith`
to let it work even if there's extra spaces in the command, e.g.

    git  checkout unknown

tests/rules/test_git_checkout.py

@@ -52,7 +52,7 @@ def test_get_branches(branches, branch_list, git_branch):
 @pytest.mark.parametrize('branches, command, new_command', [
     (b'',
      Command('git checkout unknown', did_not_match('unknown')),
-     'git branch unknown && git checkout unknown'),
+     'git checkout -b unknown'),
     (b'',
      Command('git commit unknown', did_not_match('unknown')),
      'git branch unknown && git commit unknown'),

thefuck/rules/git_checkout.py

@@ -34,6 +34,8 @@ def get_new_command(command):
                                        fallback_to_first=False)
     if closest_branch:
         return replace_argument(command.script, missing_file, closest_branch)
+    elif command.script_parts[1] == 'checkout':
+        return replace_argument(command.script, 'checkout', 'checkout -b')
     else:
         return shell.and_('git branch {}', '{}').format(
             missing_file, command.script)