Narrow down docblock type of Deferred::promise

[Gert-dev]

The interface indicates it returns a PromiseInterface, but the implementation returns a Promise, which also implements ExtendedPromiseInterface.

Calling Deferred::promise() will make PHPStan complain if you need to return an ExtendedPromiseInterface, as it cannot deduce that it does indeed return it from the implementation alone.

This should also aid autocompletion in IDEs.

[Gert-dev]

I didn't consider that, but you are right; it is a subtle BC break.

I see that you indeed added PromiseInterface on the master branch and the ExtendedPromiseInterface doesn't appear to exist anymore, so this becomes a nonissue there. I'll close this pull request in that case, thanks!