Add support for fast-forward PR merging for GitHub

[kipz]

GitLab has it: https://docs.gitlab.com/ee/user/project/merge_requests/fast_forward_merge.html

GitHub does not support it.

Rough process is:

• If skill is configured to "attempt PR fast-forward merge", then
• git clone <repo>
• git checkout <default>
• git merge <pr-branch> --ff-only # will fail if ff not possible
• git push # will fail if origin has moved forward

If this fails, we need to rebase, squash or do a normal merge commit, probably via GitHub apis.

[atomist[bot]]

Thanks for your contribution!

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