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