Ability to disable pull requests auto-updating

Recently we've noticed several old open PRs get updated without any human intervention.

This is not the best behavior given our development workflow, which is based around pull requesting from anonymous heads in a mercurial repository and does not use feature branches.

Worse, it appears that pull requests do not actually auto-update at all, instead all pending PRs get auto-updated when another pull request is merged.

For example, last night the following pull requests was merged:

https://bitbucket.org/enzo/enzo-dev/pull-request/206/adding-rotating-sphere-test-problem/activity

This caused the following pull requests to be automatically updated:

https://bitbucket.org/enzo/enzo-dev/pull-request/173/fixes-header-include-order-and-missing/activity

https://bitbucket.org/enzo/enzo-dev/pull-request/199/keep-initially-uniform-metallicity-fields/diff

https://bitbucket.org/enzo/enzo-dev/pull-request/202/adding-the-agora-isolated-disk-problem-and/diff

In all three cases this was not the desired behavior.

I don't think it's a good idea to let pull requests be updated without some sort of manual confirmation from the PR issuer. I would appreciate a way to revert to the old behavior.