The attached file, playground3.rextile, contains a session that provokes a 
situation where hg tries to merge two file revs, one of which is an ancestor 
of the other, but fails to merge because the chosen changelog ancestor lists 
an even older rev. As per mpm's analysis of my last case, there is an 
equidistant other changelog ancestor which would work, but that is not the 
point here. I think Hg should just pick the descendant when merging two files 
where one is a descendant of the another. But maybe there are equally valid 
scenarios where this would not be the right thing to do. Ideas?