Label use to enable the auto-rebase skill on a PR

[slimslenderslacks]

Question: can you currently put [auto-rebase:on-push] in a comment somewhere to automatically request that PR be rebased on Push?

• auto-rebasing seems particularly useful when the PR branch HEAD has the base branch HEAD as it's immediate parent, AND the commit was done by Atomist. In that case, the auto-rebase just integrates the changes early (it's kind of a proactive rebase and merge strategy). We could consider making this skill try to detect these cases and then be configured to not require a Label for that.

The requirement to add the label is listed in the Readme right now but I'm not sure it's a flow that will feel very natural to users. Making a change with a commit message that asks Atomist to turn this on is interesting though.

[cdupuis]

We can add commit message and comment processing to detect that rebase tag similar to auto merge.