"Upgrade" Pull request diffs opened from the "Pull Requests" view after the PR is checked out

[alexr00]

To reproduce the problem:

1. Go to the Pull Requests view.
2. Expand a Pull Request there. Open a diff. Keep this diff open, make sure it's not a preview (ctrl + K, ctrl + enter), and put your cursor in another editor so that the diff is no longer the active editor.
3. Decide you now want to check out the PR and right click -> checkout PR.
4. From your tabs or your currently open editors, you navigate back to the diff in step 2.
5. Intellisense does not work in this diff. This is happening because the diff came from the "Pull Requests" view instead of the "Changes in Pull Request" view.

This is confusing and unintuitive behavior. We should "upgrade" or close (+ maybe reopen) the already open diff so that users don't end up in what appears to be a broken state.

[alexr00]

The new Tabs API will let us close the readonly editors, but we can't replace them with the editable editors. We can only open the editable editors, not specify a locaction.

For now, this will be better than the confusing alternative. If new Tabs API is added later than enables replacing then we will switch over to that.