Avoid use of float arithmetic in bipartite_match.c.

 Avoid use of float arithmetic in bipartite_match.c.
 
 Since the distances used in this algorithm are small integers (not more
 than the size of the U set, in fact), there is no good reason to use float
 arithmetic for them.  Use short ints instead: they're smaller, faster, and
 require no special portability assumptions.
 
 Per testing by Greg Stark, which disclosed that the code got into an
 infinite loop on VAX for lack of IEEE-style float infinities.  We don't
 really care all that much whether Postgres can run on a VAX anymore,
 but there seems sufficient reason to change this code anyway.
 
 In passing, make a few other small adjustments to make the code match
 usual Postgres coding style a bit better.