[JENKINS-23365] Follow-up API fixes: introduce SCM.getKey(), and add an SCMRevisionState baseline argument to checkout.
Necessary in order to produce correct changelogs by SCM plugins which actually use SCMRevisionState correctly (unlike git-plugin!).

Corrects 9ce6b0fe.

Existing code referring to a member of type AbstractBuild often called methods not present in Run.
Changed to retain the existing fields and getters but deprecating them and introducing replacements.

Fix of JENKINS-8408 broke some tests of workspace-based polling; disable the fix when inside a test, for better predictability.
(Ideally Jenkins would actually detect whether there was a *plan* to connect a slave of a given name.)

Documented this feature and integrated a newer version of Stapler.

I took the opportunity to reorder the content in the section, so that
the tree parameter becomes the main subject.

At this point, the depth parameter should be only used in a limited
circumstance, so it should be treated as such.

I came into this use case while working on the workflow, but I think it's useful in other context as well,
such as Scriptler plugin needing to define some parameters.

Make sure the RepositoryBrowser is associated with a ChangeLogSet so we do not need to go back through the build and SCM to find it.

Added annotations to kernel security classes.