Give a group-based name to magic-key buffers

* magit-key-mode.el: Give a specific name to magic-key buffers according to group used.
(magit-key-mode-buf-name): Now a format string.
(magit-key-mode-last-buffer): New internal var.
(magit-key-mode): Create a new buffer name specifying the group in use.
(magit-key-mode-kill-buffer): Kill magit-key-mode-last-buffer.
Remove a defvar declaration

* magit-key-mode.el (magit-log-mode-window-conf): Remove one defvar declaration which is already defined at top level.