We now report a distinct buildset result to the database if the upstream code
review system is unable to merge a change. Previously it was reported as
MERGE_CONFLICT which makes it difficult to distinguish from merge conflicts.
Essentially, the two states we're interested in are when Zuul's merger is
unable to prepare a git checkout of the change (99% of the time, this is
a merge conflict). This is the MERGE_CONFLICT result.
The second state is when Zuul asks Gerrit/Github/etc to submit/merge a change
and the remote system is unable (or refuses) to do so. This is MERGE_FAILURE.
Change-Id: I47af242aeb19ae4b4aedde353fcc76ff82d90fbe
370d36c025