On the one hand, merge requests are the same as retag-requests. So it makes sense that questions asking about tags to be merged should get retag-request.
However, there are also questions about the tag merge utility. retagging is itself a synonym of tags, which makes sense because questions about retagging practices can be simplified as questions about tags. The same cannot be said about merging, which is a bigger subject than what tags conveys.
Could we break the synonym? Maybe instate a tag-merge-request tag that is synonymized with retag-request, that way people who initially think of tagging a merge request with the merging tag will see the request option. As it stands, even if we provide an alternate tag like, say, merge-tags, people will still see tag-merge in the suggestions when asking about the merge utility, so it'll just inspire a greater ease in picking the wrong tag.