Don't let the partially loaded Jenkins cause a possible data loss.

Revisited the abstraction. Allowing this method to return a mutable list, following the trend in the core to expose intelligent collection (as opposed to have the parent define CRUD methods.)

Expose a new beforeUse callback on WorkspaceListener for performing actions on a workspace before it's used by a build.

[FIX JENKINS-10536] Package Groovy DSL descriptor for eclipse in core jar to ease creation of groovy views in eclipse

CLOUDBEES-2124.

instead of the default HTML table look. Also moving the
"Configuration Matrix" title as a part of the table, to gain some space.

- doc update

In the JavaDocs for SCM#calcRevisionsFromBuild, if says:
This method is called after source code is checked out for the given build (that is, after SCM#checkout has finished successfully.)

This appears to make sense; after you have a known SCM state, you calculate the revisions.  It doesn't explicitly state it, but it
implies that it won't be called if checkout failed.  In those cases, the state may well not be known, and it doesn't make sense to
attempt to calculate the revisions.

The behavior in AbstractProject#checkout has been to always call calcPollingBaseline, regardless of whether checkout failed or
succeeded.  This is causing misleading error output for the mercurial plugin.  With this change, AbstractProject will only do the
calculations when checkout succeeds.

made the code more robust (against SCM computing null change set), and fixed the race condition (if GC happens during this operation.)