Support blame mode for revision files and add command to show item on previous revision

These two patches are together because they deserve the same purpose:
Doing git blame by going into multiple revision steps when you want to blame code that as just been moved.

So we add the capacity to blame buffers that show a file under a specified revision (got by magit-show)
And we add a command magit-visit-item-before-commit which, when used in a magit-commit buffer, open the current item at the revision just before the current commit.