How I can use git flow release finish in a manner that doesn\'t ask for merge commit messages? The -m flag doesn\'t provide this, as I expected.
git flow is (more or less) just a wrapper around git commands. Pop open the git-flow-*.sh of your choice, copy the git commands, and then mess with them. In this case, add --no-edit to git merge. This approach kills two birds with one stone: you can just copy these lines into your automation script.