Steve Losh noted on Twitter that there's no usable URL to pull changes from pull request in your command line. Well, there is, sorta, pull request page html contain hidden "merge-commands" <pre> in case there are conflicts, all you have to do is show it.

I used greasemonkey to add "hg" button between "merge" and "edit" buttons on pull request page. It looks like this:

Here is script source: