Use regular dict in get_reverse_bindings_for.

Returning a defaultdict made the caller's code look confusing, as it
wasn't clear why there wouldn't be a Keyerror in some cases. Instead,
let the caller explicitly use `get`.

qutebrowser/completion/models/miscmodels.py

@@ -278,7 +278,7 @@ def _get_cmd_completions(include_hidden, include_aliases, prefix=''):
         hide_debug = obj.debug and not objreg.get('args').debug
         hide_hidden = obj.hide and not include_hidden
         if not (hide_debug or hide_hidden or obj.deprecated):
-            bindings = ', '.join(cmd_to_keys[obj.name])
+            bindings = ', '.join(cmd_to_keys.get(obj.name, []))
             cmdlist.append((prefix + obj.name, obj.desc, bindings))
 
     if include_aliases:

qutebrowser/config/parsers/keyconf.py

@@ -424,8 +424,9 @@ class KeyConfigParser(QObject):
 
     def get_reverse_bindings_for(self, section):
         """Get a dict of commands to a list of bindings for the section."""
-        cmd_to_keys = collections.defaultdict(list)
+        cmd_to_keys = {}
         for key, cmd in self.get_bindings_for(section).items():
+            cmd_to_keys.setdefault(cmd, [])
             # put special bindings last
             if utils.is_special_key(key):
                 cmd_to_keys[cmd].append(key)