Use standard diff separator for regression.diffs

Instead of ======..., use the standard separator for a multi-file
diff, which is, per POSIX,

    "diff %s %s %s\n", <diff_options>, <filename1>, <filename2>

This makes regression.diffs behave more like a proper diff file, for
use with other tools.  And it shows the diff options used, for
clarity.

Discussion: https://www.postgresql.org/message-id/70440c81-37bb-76dd-e48b-b5a9550d5613@2ndquadrant.com

src/test/regress/pg_regress.c

@@ -1454,20 +1454,23 @@ results_differ(const char *testname, const char *resultsfile, const char *defaul
 	 * Use the best comparison file to generate the "pretty" diff, which we
 	 * append to the diffs summary file.
 	 */
-	snprintf(cmd, sizeof(cmd),
-			 "diff %s \"%s\" \"%s\" >> \"%s\"",
-			 pretty_diff_opts, best_expect_file, resultsfile, difffilename);
-	run_diff(cmd, difffilename);
 
-	/* And append a separator */
+	/* Write diff header */
 	difffile = fopen(difffilename, "a");
 	if (difffile)
 	{
 		fprintf(difffile,
-				"\n======================================================================\n\n");
+				"diff %s %s %s\n",
+				pretty_diff_opts, best_expect_file, resultsfile);
 		fclose(difffile);
 	}
 
+	/* Run diff */
+	snprintf(cmd, sizeof(cmd),
+			 "diff %s \"%s\" \"%s\" >> \"%s\"",
+			 pretty_diff_opts, best_expect_file, resultsfile, difffilename);
+	run_diff(cmd, difffilename);
+
 	unlink(diff);
 	return true;
 }