pg_rewind: Fix thinko in parsing target WAL.

It's entirely possible to see WAL for a relation that doesn't exist in
the target anymore. That happens when the relation was dropped later.
The refactoring in commit eb00f1d4 broke that case, by sanity-checking
the file type in the target before checking the flag forwhether it
exists there at all.

I noticed this during manual testing. Modify the 001_basic.pl test so
that it covers this case.

src/bin/pg_rewind/filemap.c

@@ -324,11 +324,13 @@ process_target_wal_block_change(ForkNumber forknum, RelFileNode rnode,
 	{
 		Assert(entry->isrelfile);
 
+		if (entry->target_exists)
+		{
 			if (entry->target_type != FILE_TYPE_REGULAR)
 				pg_fatal("unexpected page modification for non-regular file \"%s\"",
 						 entry->path);
 
-		if (entry->target_exists && entry->source_exists)
+			if (entry->source_exists)
 			{
 				off_t		end_offset;
 
@@ -337,6 +339,7 @@ process_target_wal_block_change(ForkNumber forknum, RelFileNode rnode,
 					datapagemap_add(&entry->target_pages_to_overwrite, blkno_inseg);
 			}
 		}
+	}
 }
 
 /*

src/bin/pg_rewind/t/001_basic.pl

@@ -71,6 +71,7 @@ sub run_test
 	primary_psql("VACUUM tail_tbl");
 
 	# Drop drop_tbl. pg_rewind should copy it back.
+	primary_psql("insert into drop_tbl values ('in primary, after promotion')");
 	primary_psql("DROP TABLE drop_tbl");
 
 	# Before running pg_rewind, do a couple of extra tests with several