Calculate the list of interesting files for a merge.

Calculate the list of interesting files for a merge.

If there is a GCA for the parents of the merge, then a file is
interesting if some parent has a version that is different from both
the child and the GCA.  If there is no GCA (e.g. for a merge that
pulls in an external project) then a file is interesting if the child's
version is different from all of the parents.

Next step is to actually show the differences for the interesting
files...