Refactor magit-git-output and magit-show-commit

The command ``magit-git-output`` has been removed in recent version of magit (say magit-20140825.940 from MELPA). This command uses ``magit-git-insert`` to accomplish the same thing.

Another change is that ``magit-show-status`` no longer accepts 3 args; now it accepts only one. ``t`` here means that the buffer showing gerrit diff will be selected (rather than the Gnus buffer).