* 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.