Include more useful information in the "still building" message

[emilyalbini]

The current message the bot posts while building is something like this:

Build {uuid} is still pending...

I'd include more useful information in that message, such as:

Build of {git-ref} for {app} is still pending...

That way it's easier to see what's happening at a glance, without having to scroll up and check the initial message.