Avoid updating the block gap when it's unchanged

Previously, the block gap storage could be updated even when the gap is
the same as before. This patch fixes it by only updating the storage
when the gap is changed.

substrate/client/db/src/lib.rs

@@ -1693,19 +1693,19 @@ impl<Block: BlockT> Backend<Block> {
 							number,
 							hash,
 						)?;
-					}
-					if start > end {
-						transaction.remove(columns::META, meta_keys::BLOCK_GAP);
-						block_gap = None;
-						debug!(target: "db", "Removed block gap.");
-					} else {
-						block_gap = Some((start, end));
-						debug!(target: "db", "Update block gap. {:?}", block_gap);
-						transaction.set(
-							columns::META,
-							meta_keys::BLOCK_GAP,
-							&(start, end).encode(),
-						);
+						if start > end {
+							transaction.remove(columns::META, meta_keys::BLOCK_GAP);
+							block_gap = None;
+							debug!(target: "db", "Removed block gap.");
+						} else {
+							block_gap = Some((start, end));
+							debug!(target: "db", "Update block gap. {:?}", block_gap);
+							transaction.set(
+								columns::META,
+								meta_keys::BLOCK_GAP,
+								&(start, end).encode(),
+							);
+						}
 					}
 				} else if number > best_num + One::one() &&
 					number > One::one() && self.blockchain.header(parent_hash)?.is_none()