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

[Johannes Sixt <j6t@kdbg.org>]

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