Conflicts where left == right

[slarse]

Due to the conservative XY-matcher used for the left-to-right matching, identical additions can sometimes cause very strange conflicts. For example, given these three revisions of a method call:

left	base	right
add(-a, b, c)	add(a, b)	sum(-a, b, c)

We get the following merge conflict:

sum(-a, b, 
<<<<<<< LEFT
c
=======
c
>>>>>>> RIGHT
        );

The reason for this is that the c node is, in fact, not matched by the XY-matcher. Regardless of how we tune the matcher, there's always a slight possibility of this happening. A simple solution would be to add a conflict handler that simply checks if the left and right sides of a conflict are equal, and if they are just pick either side as the resolution.