During a recent large merge, m-cMerge kept hanging. Ed eventually had to comment on some bugs multiple times, as it was difficult to calculate how far the previous attempt had got (not to mention how laborious it would have been to click all the comment checkboxes to deselect).
We need an optional URL parameter to force m-cMerge to download the comments for bugs, and disable commenting by changeset+bug if a comment containing the changeset URL is already present in the given bug.
We don't want this enabled by default due to the higher load on BZAPI, longer loading times, and ballooning memory usage for large merges.
In IRC, Ed suggested ?resume=1. I concur.