Only diminish "magit-auto-revert-mode" if registered
Only diminish "magit-auto-revert-mode" if registered

With a vanilla clone of exordium to `~/.emacs.d` , I found that the init phase hit issues (Emacs 24.4.1).

`emacs --debug-init` showed this to be a problem in `require(init-git)`:
`signal(error ("magit-auto-revert-mode is not currently registered as a minor mode"))`

The diff should diminish `magit-auto-revert-mode` only when registered. With the change, the init phase succeeded for a vanilla `.emacs.d` clone.
(I didn't have a case where "MRev" was shown in the modeline for comparison, but the method was http://stackoverflow.com/questions/10088168/how-to-check-whether-a-minor-mode-e-g-flymake-mode-is-on )
dvamin/dot.emacs,steve-downey/dot.emacs,jbibollet/exordium,psipika/exordium,fcassirer/dot.emacs,philippe-grenet/exordium,pkryger/exordium