Provide a 'rebase' option

[laeubi]

In the Jenkins Github Plugin one could choose an option that reads:

Merging the pull request with the current target branch revision

Discover each pull request once with the discovered revision corresponding to the result of merging with the current revision of the target branch.

it would be useful if the checkout action could also has such a "rebase" option to make sure if a build is performed it is always done on the rebased result of the target (if they diverged).