magit-revert-buffers: perform cheap tests first
magit-key-mode: don't delete other windows to show menu

Conflicts:
	magit-key-mode.el
magit-key-mode-command: kill key-mode buffer before calling command
magit-key-mode-command: cleanup
magit-key-mode-add-option: inverse test
sanitize tests/run-tests.el

* don't use defvar when let is enough
* use built-in functions to create paths
* add /path/to/magit/../DEPENDENCY to load-path
* test whether dirs exist before adding to load-path
tests/run-tests.el: /path/to/magit/../DEPENDENCY to load-path