Avoid moving data directory in upgrade testing.

Windows sometimes gets upset if we rename a large directory and then try
to use the old name quickly, as seen in occasional buildfarm failures.
So we avoid that by building the old version in the intended
destination in the first place instead of renaming it, similar to the
change made for the same reason in commit b7f8465c.

contrib/pg_upgrade/test.sh

@@ -62,9 +62,10 @@ newsrc=`cd ../.. && pwd`
 PATH=$bindir:$PATH
 export PATH
 
-PGDATA=$temp_root/data
+BASE_PGDATA=$temp_root/data
+PGDATA="$BASE_PGDATA.old"
 export PGDATA
-rm -rf "$PGDATA" "$PGDATA".old
+rm -rf "$BASE_PGDATA" "$PGDATA"
 
 unset PGDATABASE
 unset PGUSER
@@ -120,7 +121,7 @@ if [ -n "$pg_dumpall1_status" ]; then
 	exit 1
 fi
 
-mv "${PGDATA}" "${PGDATA}.old"
+PGDATA=$BASE_PGDATA
 
 initdb