Fix pgindent --show-diff option.

At least on my machine, the initial coding of this didn't actually
work, because interpolation of "$post_fh->filename" doesn't act
as intended.

I threw in some double quotes too, just in case anybody tries
to run this in a path containing spaces.

src/tools/pgindent/pgindent

@@ -299,12 +299,13 @@ sub show_diff
 	my $source_filename = shift;
 
 	my $post_fh = new File::Temp(TEMPLATE => "pgdiffXXXXX");
+	my $post_fh_filename = $post_fh->filename;
 
 	print $post_fh $indented;
 
 	$post_fh->close();
 
-	my $diff = `diff -upd $source_filename $post_fh->filename  2>&1`;
+	my $diff = `diff -upd "$source_filename" "$post_fh_filename"  2>&1`;
 	return $diff;
 }