CI-agnostic way to display logs on failure

[krlmlr]

Travis CI: Default to print to job log AppVeyor: Capable of hosting artifacts

[krlmlr]

Push artifacts to an orphan branch, tag it, print link to console.

[krlmlr]

Can be achieved (mostly) with existing code, create a demo package to illustrate this.

• step_rcmdcheck() needs to keep check output (optionally or only when failing)
• setup SSH keys for the "after_failure" stage, use a branch name generated from ci()$get_build_number()

Orphan branches are problematic because they will be downloaded by default when the user does git fetch . But maybe that's not much of a problem if only logs for failed builds are pushed.

[pat-s]

Do we really want this? Aren't we fine with uploading artifacts to the CI provider and having this set in the default templates we're shipping?

[krlmlr]

Maybe we can make AppVeyor uploads work again?

[pat-s]

Yes, we should/could do that