Don't try to backfill if limit is zero

mautrix_telegram/example-config.yaml

@@ -355,6 +355,9 @@ bridge:
         normal_groups: false
 
         # Forward backfilling limits. These apply to both MSC2716 and legacy backfill.
+        #
+        # Using a negative initial limit is not recommended, as it would try to backfill everything in a single batch.
+        # MSC2716 and the incremental settings are meant for backfilling everything incrementally rather than at once.
         forward:
             # Number of messages to backfill immediately after creating a portal.
             initial_limit: 10

mautrix_telegram/portal.py

@@ -2646,6 +2646,8 @@ class Portal(DBPortal, BasePortal):
     ) -> str:
         type = "initial" if initial else "sync"
         limit = override_limit or self.config[f"bridge.backfill.forward.{type}_limit"]
+        if limit == 0:
+            return "Limit is zero, not backfilling"
         with self.backfill_lock:
             output = await self.backfill(
                 source, source.client, forward=True, forward_limit=limit, last_tgid=last_tgid