call external editor from git-gui?

git-gui is very good at composing commits. But for writing the actual commit message -- especially if it's long and needs paragraph reflowing, indentation, and so on -- I vastly prefer emacs over git-gui's text field.

It seems to me like it would be straightforward to have a button or something to launch the user's favorite editor for editing of the commit message -- after all, git even knows what editor that is!

Unfortunately, with my tcl expertise, this is likely a whole-weekend project, so I probably won't try to build it myself in the near future.