Add git-timemachine support to browse-at-remote

browse-at-remote commands will now open the current file at the active
revision on that project's git forge in your browser.

modules/emacs/vc/config.el

@@ -48,6 +48,31 @@
   ;; `header-line-format', which has better visibility.
   (setq git-timemachine-show-minibuffer-details t)
 
+  ;; TODO PR this to `git-timemachine'
+  (defadvice! +vc-support-git-timemachine-a (orig-fn)
+    "Allow `browse-at-remote' commands in git-timemachine buffers to open that
+file in your browser at the visited revision."
+    :around #'browse-at-remote-get-url
+    (if git-timemachine-mode
+        (let* ((start-line (line-number-at-pos (min (region-beginning) (region-end))))
+               (end-line (line-number-at-pos (max (region-beginning) (region-end))))
+               (remote-ref (browse-at-remote--remote-ref buffer-file-name))
+               (remote (car remote-ref))
+               (ref (car git-timemachine-revision))
+               (relname
+                (file-relative-name
+                 buffer-file-name (expand-file-name (vc-git-root buffer-file-name))))
+               (target-repo (browse-at-remote--get-url-from-remote remote))
+               (remote-type (browse-at-remote--get-remote-type target-repo))
+               (repo-url (cdr target-repo))
+               (url-formatter (browse-at-remote--get-formatter 'region-url remote-type)))
+          (unless url-formatter
+            (error (format "Origin repo parsing failed: %s" repo-url)))
+          (funcall url-formatter repo-url ref relname
+                   (if start-line start-line)
+                   (if (and end-line (not (equal start-line end-line))) end-line)))
+      (funcall orig-fn)))
+
   (defadvice! +vc-update-header-line-a (revision)
     "Show revision details in the header-line, instead of the minibuffer.