Avoid some missed updates when backtracking.

Avoid some missed updates when backtracking.

This avoids unnecessarily missed updates like the following:

WARNING: One or more updates have been skipped due to a dependency conflict:

dev-util/boost-build:0

  (dev-util/boost-build-1.53.0::gentoo, ebuild scheduled for merge) conflicts with
    =dev-util/boost-build-1.52.0-r1 required by (dev-libs/boost-1.52.0-r6::gentoo, installed)

!!! The following update(s) have been skipped due to unsatisfied dependencies
!!! triggered by backtracking:

dev-libs/boost:0