I use a a more complicated command. As far as I remember I got it from this thread http://kerneltrap.org/mailarchive/git/2007/6/28/250230 (probably the same as what you are referring to).
[mergetool.ediff]
cmd = emacs --eval \"\
(progn\
(defun ediff-write-merge-buffer ()\
(let ((file ediff-merge-store-file))\
(set-buffer ediff-buffer-C)\
(write-region (point-min) (point-max) file)\
(message \\\"Merge buffer saved in: %s\\\" file)\
(set-buffer-modified-p nil)\
(sit-for 1)))\
(setq ediff-quit-hook 'kill-emacs\
ediff-quit-merge-hook 'ediff-write-merge-buffer)\
(ediff-merge-files-with-ancestor \\\"$LOCAL\\\" \\\"$REMOTE\\\"\
\\\"$BASE\\\" nil \\\"$MERGED\\\"))\"
Note that I have split this across several lines to increase readability and escaped the newline with \ so git config considers it as a single line.
I usually use emacsclient to edit e.g. commit messages. The above mergetool configuration unfortunately does not use emacsclient, and when I tried to get it to work with emacsclient I ran in to various problems including the fact that emacsclient returned right away.
But you just reminded me of that issue, so I might work on fixing that problem soon. However if someone else already found a solution that would be great of course 😉