eval/nixpkgs: parse more commit messages

[rhendric]

Add support for expanding the Bash-like brace syntax and for removing bracketed prefixes like [Backport ...] [that was stupid; those are found in the PR titles, not the commit messages].

[Artturin]

library seems simple enough and the tests pass

[asymmetric]

I would suggest making an announcement on Discourse if this gets merged, otherwise people might miss it.

[cole-h]

I just pushed a small commit (6af23b6) that adds a note why the format! call is necessary.

I'll merge and deploy this later today (just to make sure that my previous deploy of #650 works as I expect).

[cole-h]

(To expand on my hand-wavey "later today", here's an explicit time:) I expect I'll merge and deploy this around 11AM Pacific (in roughly 4 hours).