Pull requests unapproved when sync with destination branch to an unrelated file

It would be a useful feature if the pull requests would still be approved when we do a sync with the destination branch to an unrelated file.

Steps to reproduce:

1) Create a new branch and make some changes to a single file. Commit and push.
2) Create pull request to merge to the `default' branch.
3) Approve PR.
4) Make some changes to an unrelated file on the `default` branch. Commit and push.
5) Select `Sync now` on the PR to bring the new branch up to date with the `default` branch.
6) PR state changes to `Unapproved` - which is actually the expected behaviour as the source branch has changed.