Don't remove dependent version when downgrading to a version.

Don't remove dependent version when downgrading to a version.

Adjusted the version traversal on downgrade
such that we can downgrade to a version that is a dependency for
a version in a different branch, *without* needing to remove that
dependent version as well.  Previously, the target version would be
seen as a "merge point" for it's normal up-revision as well as the
dependency.  This integrates with the changes for :ticket:`377`
and :ticket:`378` to improve treatment of branches with dependencies
overall.

Change-Id: Ica0732f6419f68ab85650170839ac8000ba3bbfb
Fixes: #379