support aborting a merge action

Suggested-By: Grégoire Jadi <gregoire.jadi@gmail.com>
Revert "magit-key-mode-popup-merging: add --no-commit"

This reverts commit 2d6f9ffca9bc18d4da23e40a522c336d8ad53f60.

Unfortunatly that isn't enough. The various `magit-*-merge'
should be merged into one.