magit-describe-item: add battle proved implementation

That's what I used when I last worked on the section code.
I just never got to move this from by init file into Magit.
add magit-with-blob
magit-font-lock-keywords: remove obsolete entries
magit-tests.el: don't require magit-blame

Earlier the only test related to magit-blame has been removed.
magit-tests.el: don't require cl

We no longer use anything from cl; and if we did, we should be using
cl-lib instead.