pg_regress: Allow overriding diff options

By setting the environment variable PG_REGRESS_DIFF_OPTS, custom diff
options can be passed.

reviewed by Jeevan Chalke

doc/src/sgml/regress.sgml

@@ -285,7 +285,10 @@ gmake check EXTRA_TESTS=collate.linux.utf8 LANG=en_US.utf8
     file against the reference outputs stored in the
     <filename>src/test/regress/expected</filename> directory.  Any
     differences are saved for your inspection in
-    <filename>src/test/regress/regression.diffs</filename>.  (Or you
+    <filename>src/test/regress/regression.diffs</filename>.  If you don't
+    like the <command>diff</command> options that are used by default, set the
+    environment variable <envar>PG_REGRESS_DIFF_OPTS</envar>, for
+    instance <literal>PG_REGRESS_DIFF_OPTS='-u'</literal>.  (Or you
     can run <command>diff</command> yourself, if you prefer.)
    </para>
 

src/test/regress/pg_regress.c

@@ -1970,6 +1970,9 @@ regression_main(int argc, char *argv[], init_function ifunc, test_function tfunc
 	 */
 	ifunc();
 
+	if (getenv("PG_REGRESS_DIFF_OPTS"))
+		pretty_diff_opts = getenv("PG_REGRESS_DIFF_OPTS");
+
 	while ((c = getopt_long(argc, argv, "hV", long_options, &option_index)) != -1)
 	{
 		switch (c)