Currently, Diffusion parses Fixes T123 from commit summaries. However, if there's an associated revision and that revision has Fixes T123 in comments, that should also take effect. This probably depends on formalizing Fixes relationships, maybe T3906.
This is from a million years ago and I now think we should only respect commit messages. If "Fixes Txxx" works or ever worked in comments, I'd say we should actually undo that.