bug#21526: 24.5; prolog-mode: broken indentation for if-then-else construct

[Markus Triska <triska@metalevel.at>]

Here is a further test case I found in connection with if-then-else,
using your latest patch and the settings I posted after emacs -Q:

   test :-
           (a
           ;   b
           ).

When I place point on the "a" and press TAB, the expected result is:

   test :-
           (   a
           ;   b
           ).

However, in contrast to the prolog.el by Stefan Bruda, I get again:

   test :-
           (a
           ;   b
           ).

Thank you and all the best!
Markus