Detect downgrades over dependencies distinctly from unmerge

Detect downgrades over dependencies distinctly from unmerge

Previously, a downgrade to a version that is also a dependency
to another branch, where that branch is advanced beyond
the target, would fail to be downgraded, as this would
be detected as an "umerge" even though the target version
to be INSERTed would not be present.
The patch replaces the existing heuristic
that checks for "delete" with a new one that calculates
a potential "unmerge" fully, and returns False only if we in
fact could do an unmerge.

Also change the string display of a version so that we don't
display misleading target versions that might not actually
be getting invoked.

Fixes: #377
Change-Id: I7420dd7adbd9ccf9ca85b56d9a792a85c40f3454