Probably introduced in rP3dd92a78ac2a (other commits since our last update don't seem to touch this code).
When someone pushes their commit, the revision gets closed, as usual:
> xxx added a commit: rXXabcdefabcdef: Use xx for xx to fix xx
However, it now also generates a mention:
> xxx mentioned this in: rXXabcdefabcdef: Use xx for xx to fix xx
It seems a bit double. It doesn't bother me, but some coworkers were somewhat confused and thought they did something wrong.
(The commit(s) was not changed in any way after running `arc amend`)