Use 0-based numbering in comments about backup blocks.

The macros and functions that work with backup blocks in the redo function
use 0-based numbering, so let's use that consistently in the function that
generates the records too. Makes it so much easier to compare the
generation and replay functions.

Backpatch to 9.0, where we switched from 1-based to 0-based numbering.

src/backend/access/nbtree/nbtinsert.c

@@ -1221,7 +1221,7 @@ _bt_split(Relation rel, Buffer buf, OffsetNumber firstright,
 			item = (IndexTuple) PageGetItem(origpage, itemid);
 			lastrdata->data = (char *) item;
 			lastrdata->len = MAXALIGN(IndexTupleSize(item));
-			lastrdata->buffer = buf;	/* backup block 1 */
+			lastrdata->buffer = buf;	/* backup block 0 */
 			lastrdata->buffer_std = true;
 		}
 
@@ -1248,7 +1248,7 @@ _bt_split(Relation rel, Buffer buf, OffsetNumber firstright,
 
 			lastrdata->data = (char *) newitem;
 			lastrdata->len = MAXALIGN(newitemsz);
-			lastrdata->buffer = buf;	/* backup block 1 */
+			lastrdata->buffer = buf;	/* backup block 0 */
 			lastrdata->buffer_std = true;
 		}
 		else if (ropaque->btpo.level == 0)
@@ -1257,14 +1257,14 @@ _bt_split(Relation rel, Buffer buf, OffsetNumber firstright,
 			 * Although we don't need to WAL-log the new item, we still need
 			 * XLogInsert to consider storing a full-page image of the left
 			 * page, so make an empty entry referencing that buffer. This also
-			 * ensures that the left page is always backup block 1.
+			 * ensures that the left page is always backup block 0.
 			 */
 			lastrdata->next = lastrdata + 1;
 			lastrdata++;
 
 			lastrdata->data = NULL;
 			lastrdata->len = 0;
-			lastrdata->buffer = buf;	/* backup block 1 */
+			lastrdata->buffer = buf;	/* backup block 0 */
 			lastrdata->buffer_std = true;
 		}
 
@@ -1297,7 +1297,7 @@ _bt_split(Relation rel, Buffer buf, OffsetNumber firstright,
 
 			lastrdata->data = NULL;
 			lastrdata->len = 0;
-			lastrdata->buffer = sbuf;	/* backup block 2 */
+			lastrdata->buffer = sbuf;	/* backup block 1 */
 			lastrdata->buffer_std = true;
 		}