Re-add locally-generated files in pg_upgrade's .gitignore and Makefile

This reverts the changes to pg_upgrade's Makefile and .gitignore done in
15b6d21.  The TAP tests run in isolation, executing pg_upgrade in
tmp_check/ in the build directory so as any files created in the
execution path (reindex_hash.sql and delete_old_cluster.{sh,bat}) are
never in the tree, so entries are not necessary in this case.  However,
not having these impacts the cleanliness of the code tree when running
./pg_upgrade directly from src/bin/pg_upgrade/.

This commit adds back to .gitignore all the files generated in the
execution path, and the Makefile rule to clean them up if they exist.

Per gripe from Tom Lane.

Discussion: https://postgr.es/m/90595.1655227384@sss.pgh.pa.us

src/bin/pg_upgrade/.gitignore

@@ -1,4 +1,8 @@
 /pg_upgrade
+# Generated by manual invocation
+/delete_old_cluster.sh
+/delete_old_cluster.bat
+/reindex_hash.sql
 # Generated by test suite
 /log/
 /tmp_check/

src/bin/pg_upgrade/Makefile

@@ -48,7 +48,8 @@ uninstall:
 
 clean distclean maintainer-clean:
 	rm -f pg_upgrade$(X) $(OBJS)
-	rm -rf log/ tmp_check/
+	rm -rf delete_old_cluster.sh log/ tmp_check/ \
+	       reindex_hash.sql
 
 check:
 	$(prove_check)