Using emacs (and magit?) to visit a file in given commit/branch/etc

The canonical way to do that in Emacs is to use VC: C-x v ~ from the file's buffer will ask you for a revision and then show that file as it was at that revision. It should work for any control system supported by VC, such as Git, Bzr, ...

• C-xvl to view the file's history.
• n and p to move between commits.
• f to visit the file as of the commit at point.

That's bound to log-view-find-revision, and if we look at the code we see the critical bit is:

(switch-to-buffer (vc-find-revision file revision)))

So we could wrap that in a custom function like so:

(defun my-vc-visit-file-revision (file revision)
  "Visit FILE as it was at REVISION."
  (interactive
   (list (expand-file-name
          (read-file-name (if (buffer-file-name)
                              (format "File (%s): " (file-name-nondirectory
                                                     (buffer-file-name)))
                            "File: ")))
         (read-string "Revision: ")))
  (require 'vc)
  (switch-to-buffer
   (vc-find-revision file revision)))

Edit: Stefan has provided a better answer, but if you liked being able to select the file as well as the revision, here's a version of my function which maintains the interactive file selection, but uses the code from vc-revision-other-window for the revision handling.

I concluded that using other-window by default does indeed make more sense, so I've done the same here -- unless you provide a prefix argument in which case it uses the current window.

(defun my-vc-visit-file-revision (file rev)
  "Visit revision REV of FILE in another window.
With prefix argument, uses the current window instead.
If the current file is named `F', the revision is named `F.~REV~'.
If `F.~REV~' already exists, use it instead of checking it out again."
  ;; based on `vc-revision-other-window'.
  (interactive
   (let ((file (expand-file-name
                (read-file-name
                 (if (buffer-file-name)
                     (format "File (%s): " (file-name-nondirectory
                                            (buffer-file-name)))
                   "File: ")))))
     (require 'vc)
     (list file (if (vc-backend file)
                    (vc-read-revision
                     "Revision to visit (default is working revision): "
                     (list file))
                  (vc-read-revision "Revision to visit: " t
                                    (or (vc-deduce-backend)
                                        (vc-responsible-backend file)))))))
  (require 'vc)
  (let ((revision (if (string-equal rev "")
                      (if (vc-backend file)
                          (vc-working-revision file)
                        (error "No revision specified for unregistered file %s"
                               file))
                    rev))
        (backend (or (vc-backend file)
                     (vc-deduce-backend)
                     (vc-responsible-backend file)))
        (visit (if current-prefix-arg
                   'switch-to-buffer
                 'switch-to-buffer-other-window)))
    (condition-case err
        (funcall visit (vc-find-revision file revision backend))
      ;; The errors which can result when we request an invalid combination of
      ;; file and revision tend to be opaque side-effects of some unexpected
      ;; failure within the backend; so we simply trap everything and signal a
      ;; replacement error indicting the assumed cause.
      (error (error "File not found at revision %s: %s" revision file)))))

I bind this command to C-xvC-f