The title says it all. It would be nice to also be able to amend the commit message of a merge changeset. (on IRC, marmoute told me there wasn't any obvious reason against it)
Fixed by http://selenic.com/repo/hg/rev/3a72c89a83ec Brodie Rao <brodie@sf.io> amend: support amending merge changesets (issue3778) (please test the fix)