By default, Git tries to open COMMIT_EDITMSG in the actual repo -- which is sane. But if the repo is on a slow disk (e.g. external USB drive, network fs etc) the editor startup / exit time can suffer.
Short of relocating the repo, or just the .git folder associated with the repo (e.g. .git file), is there any other way to make COMMIT_EDITMSG go to /tmp? I've looked at the hooks but they don't seem to be able to set the location of the file AFAICT.