Code review comment for lp:~abentley/launchpad/twistedjob-enhancements

= Summary =
Provide misc functionality for converting jobs to use Twisted.

== Proposed fix ==
Add IBranchMergeProposalJob.id and BranchMergeProposalJob.get, which when
invoked in subclasses returns the BranchMergeProposalJob with that id as an
instance of that subclass.

Add an additional message to JobRunner indicating how many jobs did
not complete.

== Pre-implementation notes ==
None of this had a pre-implementation call, though it's basically all required
for the followon branches.

== Implementation details ==
Err...

== Tests ==
bin/test -t test_job -t test_update_preview_diffs -t test_branchmergeproposals

== Demo and Q/A ==
Look at the logs produced by update_preview_diffs on
/x/launchpad.net-logs/scripts/asuka/bzrsyncd. They should have a line that
says "x IUpdatePreviewDiffSource jobs did not complete.".