The only reason we produce FETCH_HEAD is to be compatible with C Git.  The only reason C Git produces FETCH_HEAD is to carry data from `git fetch` into `git merge` during a `git pull`, but it does also act as a handy reference for "the last thing I got".  Really the only value for JGit to be writing that file is for the last part, "the last thing I got".  We'll never need to carry data from our fetch to our merge code through a local file, that's nuts.

I think its perfectly OK if we stop writing this file on less-conventional repositories.  If "the last thing I got" proves to be really useful there, we could try to update a standard ref called FETCH_HEAD via updateRef().  But again, a lot of this all predates the idea of remote tracking branches being updated during fetch.