Outsource

```
remove magit-log-edit.el and rebase-mode.el
    
    Instead use `git-commit-mode.el' and `git-rebase-mode.el'
    from https://github.com/magit/git-modes.
    
    * `git-commit-mode' communicates the commit message to git by
      saving the commit message file. `magit-log-edit' used git's
      standard input for this purpose, which is much more fragile.
    
    * `git-rebase-mode' is the new name of the library previously
      known as `rebase-mode'.  It was moved to `git-modes' because
      we start using that anyway and because, like the other files
      there, can be used without magit.
    
    This is been discussed at https://github.com/magit/magit/issues/675.
```

I am ready now.

This pull request as well as the following should be merged at about the same time.

**(This will be filled out shortly.)**

* TODO git-modes
* TODO melpa