Use bzr 'short' status for more robust parsing (closes bgo#348722)

bzr's 'short' version of the status command is easier to parse reliably
than the default version, and lets us easily ignore status changes that
we don't yet support.

For a list of possible status flags, see:
http://doc.bazaar.canonical.com/latest/en/user-reference/
status-flags-help.html