Set this-command in magit-key-mode-command

Some library (ido) could have problem when this-command is set to
something that is not a symbol, and when a key is pressed, its binding
is put into this-command. In magit-key-mode, lambda expression are bind
to key, and this could confuse ido.

As a side effect, last-command will be set to the real command that have
be run, and not to lambda expression.

The problem with ido is probably a bug in Emacs.
Merge pull request #1284 from vanicat/t/this-command-in-magit-key-mode

Set this-command in magit-key-mode-command
Merge branch 'master' into next

But keep our side.  There is no `magit-key-mode.el' in `next'
and we also don't have to implement the equivalent of
670599c147647e340740d2e48c611ee8633f647c for `magit-popup.el'.