Let forward_messages work with messages from different chats

Previously it would take the first chat it found and use the IDs
of all messages, even if they belonged to different chats, resulting
in unexpected messages to be forwarded.

Another solution would be to check that all the chats are the same,
but this solves the issue more nicely and makes it more powerful.

telethon/client/messages.py

@@ -644,30 +644,43 @@ class MessageMethods(UploadMethods, ButtonMethods, MessageParseMethods):
         if single:
             messages = (messages,)
 
-        if not from_peer:
-            try:
-                # On private chats (to_id = PeerUser), if the message is
-                # not outgoing, we actually need to use "from_id" to get
-                # the conversation on which the message was sent.
-                from_peer = next(
-                    m.from_id
-                    if not m.out and isinstance(m.to_id, types.PeerUser)
-                    else m.to_id for m in messages
-                    if isinstance(m, types.Message)
-                )
-            except StopIteration:
-                raise ValueError(
-                    'from_peer must be given if integer IDs are used'
-                ) from None
+        entity = await self.get_input_entity(entity)
+
+        if from_peer:
+            from_peer = await self.get_input_entity(from_peer)
+            from_peer_id = await self.get_peer_id(from_peer)
+        else:
+            from_peer_id = None
+
+        def get_key(m):
+            if isinstance(m, int):
+                if from_peer_id is not None:
+                    return from_peer_id
+
+                raise ValueError('from_peer must be given if integer IDs are used')
+            elif isinstance(m, types.Message):
+                return m.chat_id
+            else:
+                raise TypeError('Cannot forward messages of type {}'.format(type(m)))
+
+        sent = []
+        for chat_id, group in itertools.groupby(messages, key=get_key):
+            group = list(group)
+            if isinstance(group[0], int):
+                chat = from_peer
+            else:
+                chat = await group[0].get_input_chat()
+                group = [m.id for m in group]
+
+            req = functions.messages.ForwardMessagesRequest(
+                from_peer=chat,
+                id=group,
+                to_peer=entity,
+                silent=silent
+            )
+            result = await self(req)
+            sent.extend(self._get_response_message(req, result, entity))
 
-        req = functions.messages.ForwardMessagesRequest(
-            from_peer=from_peer,
-            id=[m if isinstance(m, int) else m.id for m in messages],
-            to_peer=entity,
-            silent=silent
-        )
-        result = await self(req)
-        sent = self._get_response_message(req, result, entity)
         return sent[0] if single else sent
 
     async def edit_message(