Libcheckout: Add support to optionally merge on PRs

[alvaroaleman]

In the case of PRs, checkout just checks out the PR. It would be nice if there was an option to also merge the PR into the base branch, so that issues caused by changes merged into the basebranch after the PR branch was crated get surfaced. This entails that PRs with conflicts won't build at all.

[lucianoliberti]

@alvaroaleman thanks, we'll analyze it and let you know