Update .gitignore for pg_upgrade

Add Windows versions of generated scripts, and make sure we only
ignore the scripts int he root directory.

Michael Paquier

contrib/pg_upgrade/.gitignore

 /pg_upgrade
 # Generated by test suite
-analyze_new_cluster.sh
-delete_old_cluster.sh
+/analyze_new_cluster.sh
+/delete_old_cluster.sh
+/analyze_new_cluster.bat
+/delete_old_cluster.bat
 /log/
 /tmp_check/