Renovate: Add PR suffix to automerge commit titles

[zephraph]

What would you like Renovate to be able to do?
When automerging a PR, add the PR # at the end of the commit message

Describe the solution you'd like
For this commit the message would be be

Update dependency @artsy/palette to v2.35.0 (#1964)

Additional context

This just makes it a little easier to trace back to the original PR.

[rarkins]

It looks like picking a customer commit title is possible via the API: https://developer.github.com/v3/pulls/#merge-a-pull-request-merge-button

Need to check if it behaves as expected for all types of commit though (merge, squash, rebase).

Then next question is whether to do this automatically or via config. Usually we want to make everything configurable.

[AhmedAbdraboh]

We can add the PR number to the options object in mergePR method with the key commit_message
https://github.com/renovatebot/renovate/blob/master/lib/platform/github/index.js#L1357
so the options can be
const options = {
commit_message: (#${prNo}),
};

[rarkins]

GitHub's API didn't behave as we expected. Therefore this might need to be done manually and also not for every merge type.

If you merge with rebase, it would have to match the commit. But you can't know the PR number before you make the commit.

If you squash merge, then it's probably possible - update the PR title and then squash merge.