Merged apt-facade-changer-fixes [r=therve,ack] [f=889067,889069,889087,889099].
Fixes to AptFacade that is needed for the package changer to work:
1) Fix mark_upgrade() to upgrade the package to the latest version,
instead of upgrading to the version that is passed in.
2) Don't do anything if you try to upgrade a package that is already
at the latest version.
3) Raise a TransactionError if something goes wrong committing the
changes to the Apt cache.
4) Take the Package object into account, when comparing Version
objects to find out which dependencies are needed for
DependencyError in perform_changes().