Encapsulate force_command in _get_raw_command

Using the `force_command` argument will run into issues as the
`_get_raw_command` method simply returns the value of `force_command`
(which is a string) while it should actually return a list.

Fix #1240

tests/entrypoints/test_fix_command.py

@@ -5,8 +5,8 @@
 
 class TestGetRawCommand(object):
     def test_from_force_command_argument(self):
-        known_args = Mock(force_command=['git', 'brunch'])
-        assert _get_raw_command(known_args) == ['git', 'brunch']
+        known_args = Mock(force_command='git brunch')
+        assert _get_raw_command(known_args) == ['git brunch']
 
     def test_from_command_argument(self, os_environ):
         os_environ['TF_HISTORY'] = None

thefuck/entrypoints/fix_command.py

@@ -12,7 +12,7 @@
 
 def _get_raw_command(known_args):
     if known_args.force_command:
-        return known_args.force_command
+        return [known_args.force_command]
     elif not os.environ.get('TF_HISTORY'):
         return known_args.command
     else: