feature req: bors queue should also link to head commit

[pnkfelix]

There is a problem with the bors UX.

It is documented on discussion here: rust-lang/rust#13157 (comment)

The summary is that github presents commits in a PR in chronological order, not topological order. So it is easy for a reviewer to mistakenly put the r+ mark on a non-head commit.

To work-around this, it would be nice if the bors queue would present a link to the head commit for a PR.

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(As a side-feature, it might also be good if bors somehow indicated if there is already an r+ mark on any commit other than the head one in the PR, so that people reviewing the queue could easily see if if a given PR has already been ("partially") reviewed and/or if the reviewer may have mistakenly put the r+ on the wrong commit.