update shadow index when doing a double-put, fixes #5661

this fixes the test added in previous commit (avoids
that the PUT reappears after index rebuild).

src/borg/repository.py

@@ -1224,10 +1224,7 @@ class Repository:
             pass
         else:
             # note: doing a delete first will do some bookkeeping.
-            # we do not want to update the shadow_index here, because
-            # we know already that we will PUT to this id, so it will
-            # be in the repo index (and we won't need it in the shadow_index).
-            self._delete(id, segment, offset, update_shadow_index=False)
+            self._delete(id, segment, offset, update_shadow_index=True)
         segment, offset = self.io.write_put(id, data)
         self.storage_quota_use += len(data) + self.io.put_header_fmt.size
         self.segments.setdefault(segment, 0)