add PR id to comments posted on commit

[pnkfelix]

It is sometimes hard to backtrack from the comments that bors leaves on a github commit over to the original Pull Request that prompted bors to attempt a merge.

It seems like bors should be able to include a link to the PR in the comment it leaves.

This would ease (for example) going to see if the author has posted a follow-up commit fixing the errors that bors found on its first merge attempt.

[pnkfelix]

as an example of what I am talking about, see here:

rust-lang/rust@bf2f84b

it would be nice if one or all of the comments from bors including a reference back over to rust-lang/rust#19964