In the topological sort for merge order, try to avoid selecting root nodes

In the topological sort for merge order, try to avoid selecting root nodes
whenever possible. This helps ensure that the maximimum possible number of
soft dependencies  have been removed from the graph before their parent nodes
have been selected. This is especially important when those dependencies are
going to be rebuilt by revdep-rebuild or `emerge -e system` after the CHOST
has been changed (like when building a stage3 from a stage2). With this patch,
`emerge -e system` properly rebuilds dev-lang/python before sys-apps/file,
which helps to avoid a potential build failure.

svn path=/main/trunk/; revision=7728