--simplify-merges breakage

Commit 4b7f53d ("simplify-merges: drop merge from irrelevant side 
branch") appears to have broken simplify-merges.

[Credit to "git bisect run" - and now I see the "report problems 
immediately" message on the recent merge of that change.]

I was trying to get my head around history simplification, by creating a 
repo with the example shown in the manual.The result of "gitk 
--simplify-merges foo" was far from pretty.

With the example in the manual, you should get this:

           .-A---M---N---O
          /     /       /
         I     B       D
          \   /       /
           `---------'

After 4b7f53 you instead get:

           .-A---M   N---O
         /     /   /   /
         I     B   /   D
          \   /   /   /
           `---------'

Kevin