Add support for rebasing fork, rather than just fast-forwarding.

[bardiharborow]

Upriver works internally by fetching upstream's sha1 hash, and resetting your forks state to match. GitHub automatically bails if doing so would lose data, but we have a "force" option to override. Instead of doing this, we might be able to work out a way to rebase the fork with upstream, but that leads us down a messy path of what do we do when we have merge conflicts. Ideas and PRs would be very appreciated.