Build commit message from tickets by default

[jlevon]

For the platform, we want the commit message to be of a specific format, namely
a list of bug synopses, without any additional commentary. We should default to doing
that, with a flag to do the existing "collate all commit messages".

[timfoster]

This seems reasonable. I'd expect a ~/.prrconfig setting as well as a CLI argument to choose "collate all commit messages" mode for people who rarely work on the platform.

I'd like it if prr could be usable by all of Joyent engineering, rather than including features that only work for a particular audience.

[jlevon]

Fixed