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.