Replace use of "diff -q".

POSIX does not specify the -q option, and many implementations do not
offer it.  Don't bother changing the MSVC build system, because having
non-GNU diff on Windows is vanishingly unlikely.  Back-patch to 9.2,
where this invocation was introduced.

Author: nmisch

contrib/pg_upgrade/test.sh

@@ -217,10 +217,11 @@ case $testhost in
 	*)	    sh ./delete_old_cluster.sh ;;
 esac
 
-if diff -q "$temp_root"/dump1.sql "$temp_root"/dump2.sql; then
+if diff "$temp_root"/dump1.sql "$temp_root"/dump2.sql >/dev/null; then
 	echo PASSED
 	exit 0
 else
+	echo "Files $temp_root/dump1.sql and $temp_root/dump2.sql differ"
 	echo "dumps were not identical"
 	exit 1
 fi