git-gui is still using old-style git-merge invocation

Performing a merge with git gui presents the following message in the merge result window:

warning: old-style 'git merge <msg> HEAD <commit>' is deprecated.
Merge made by the 'recursive' strategy.
  a | 1 +
  1 file changed, 1 insertion(+)
  create mode 100644 a

But I am unable to find where the invocation happens. Can somebody help?

-- Hannes

PS: Reproducer:

git init echo a > a git add a git commit -ma a git checkout -b side @~ echo b > b git add b git commit -mb git gui # to merge: Ctrl-M, Enter, Enter