Merging is not currently implemented. However, it will be soon, and this is the current plan:
Firstly, things like comments all have their own files and I take sufficient care over filenames to ensure you won't get a collision. For bug fields, each field has its own file. If there's a collision, there will be a nice HTML based summary which will hook into monotone's goodness for dealing sanely with 3-way merges.
The idea is just to present to the user the fact that there are multiple heads for this particular bug, what the heads are and who did them, and then let the user decide. The assumption is that only users that care will actually deal with the merges.
DisTract's approach is very lightweight: there's very little magic going on with the relationship between different field values and so forth - you can happily comment on a closed bug without reopening it for instance, so merging should come down to just choosing which particular field values you want.
You may also wish to read Fields.