There is a little bit of discussion about this here, at en.wp's VPT and it was suggested I open a ticket about it. Basically, I want "mergehistory-success" to use &redirect=no for the source page because typically after a histmerge that page is a redirect. There's a fuller explanation at the VPT link. I think this would be a useful change for all wikis, so instead of creating a local MediaWiki message at en I thought I would bring it here and see if it could be done site-wide. My limited understanding is that changing

$3 {{PLURAL:$3|revision|revisions}} of [[:$1]] successfully merged into [[:$2]].

to

$3 {{PLURAL:$3|revision|revisions}} of [{{fullurl:$1|redirect=no}} $1] successfully merged into [[:$2]].

would do the trick, but I am sure people here would know better than me.