pgsql: Replace use of "diff -q".

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.

Branch
------
REL9_3_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/d4051bfa2c4f2c1192500cf8107bc766d3854646

Modified Files
--------------
contrib/pg_upgrade/test.sh |    3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)