Mercurial pull requests with bookmarks within the same repo show "There are no changes to be pulled" (BB-14693)

With git, it's possible to push two branches to the same repo and then to create a "fake" PR between them (fake because nothing is getting pulled, it's more a "merge request" than a "pull request").

With hg, if you push two heads on the same named branch, you can't do the same, even if you identify each head by a bookmark. Everything seems to work fine at first. You can even see which commits bb thinks would be merged into the target head, but if you attempt to save the PR, bb errors out with

The following error(s) occurred saving this pull request

    There are no changes to be pulled