Assimilate git-commit.el, git-rebase.el and with-editor.el

Assimilate git-commit.el, git-rebase.el and with-editor.el into Magit. Previously these files were part of https://github.com/magit/git-modes, the first two under the names `git-commit-mode.el` and `git-rebase-mode.el`.

This move was announced here: https://github.com/magit/git-modes/issues/117.

This does not affect users who get Magit and its dependencies through Melpa yet.