Fast-track indicator in commit metadata

[jasnell]

When a pull-request has been fast-tracked, it would likely be good for us to capture that detail in the commit metadata as an FYI. I'm thinking something simple like:

PR-URL: https://.... (fast-tracked)

or

Fast-Tracked
PR-URL: https://....

Doing so would give us a permanent record just to we can make sure in case issues pop up later.

[targos]

I would avoid to change the format of the PR-URL line. It might break other tools like the changelog generator.