See PHI1555.
Revisions may be related to tasks with a `DifferentialRevisionHasTaskEdgeType::EDGECONST` edge, either via {nav Edit Related Objects... > Edit Tasks} in the web UI or by running `arc diff` on a branch named `Txxx...`. However, this relationship is not transferred to commits, so commits don't end up attached to tasks and don't show up in the (recently updated) "Related Changes" table.
For now, I plan to treat these relationships like we treat `Ref Txxx` in the commit message, and effectively copy the edge when we discover a commit. Copying edges isn't //great//, but "Ref" and "Fixes" sort of work like this already, and even if this is walked back later it isn't a big deal.
See also T5598 (which has some adjacent discussion) and T11934.