Fixing the described problem will remove this comment,
but I have no idea how better to do this. The problem is that
we need to provide own created buffer to the call to `vc-find-revision'.
Currently it has the following function signature:
(defun vc-find-revision (file revision &optional backend)
But VC API in the comments in the beginning of vc.el
is documented with a different function signature:
;; * find-revision (file rev buffer)
;;
;; Fetch revision REV of file FILE and put it into BUFFER.
;; If REV is the empty string, fetch the head of the trunk.
;; The implementation should pass the value of vc-checkout-switches
;; to the backend command.