Enable ediff support in the *hg-log-view* of emacs.

hg-ediff-init.el

;; Assuming you have already installed the 'mercurial' package in 
;; emacs, follow the steps below to enable 'ediff' support in 
;; the *hg-log-view*:
;;
;; 1) Add the following lines to your '.hgrc' such that the *hg-log-view*
;;    of emacs shows the files that have changed in the detailed information 
;;    of a changeset: 
;;
;; [ui]
;; verbose = True
;;
;; This step above is necessary until it the 'hg' command can be customized with 
;; the '-v' option in emacs. Right now this seems broken. Let me know if you 
;; find a better way.
;; 
;; 2) Copy the elisp functions below to your emacs init file (e.g. '.emacs').
;;
;; Then, to launch 'ediff' on a versioned file:
;;
;; 1) Do 'M-x vc-dir' and type the path to your hg repo. 
;; 2) In the *vc-dir* buffer, type 'l' to list 
;;    the revisions of that directory in the *hg-log-view*.
;; 3) In the *hg-log-view*, move the point to a changeset and 
;;    press 'enter' to display the files that have been modified 
;     by that changeset.
;; 4) Move the point to on one of the files in that changeset and 
;;    press 'e' to browse the changes using 'ediff'.

(add-hook 'log-view-mode-hook
          (lambda ()
            (local-set-key (kbd "e") 'log-view-ediff)))

(defun vc-hg-parent-revision (file rev)
  (with-temp-buffer
    (vc-hg-command t 0 nil "parent" "-r" rev file)
    (goto-char (point-min))
    (re-search-forward "^changeset:[ \t]*\\([0-9]+\\):")
    (match-string-no-properties 1)))

(defun log-view-ediff (&optional startup-hooks)
  (interactive)
  (let* ((file (thing-at-point 'filename))
         (to-rev (log-view-current-tag))
         (from-rev (vc-hg-parent-revision file to-rev)))
    (save-excursion
      (set-buffer (find-file-noselect file))
      (require 'ediff-vers)
      (let ((control-buffer (ediff-vc-internal from-rev to-rev startup-hooks)))
        (set-buffer control-buffer)
        (ediff-next-difference))
      (mapc (lambda (f) (funcall f)) startup-hooks))))