[FIXED JENKINS-12124] Occasionally errors loading plugin classes since it is expected that findClass (and findLoadedClass) are called under synchronization.
The problem was masked by a blind assumption that an InvocationTargetException was in fact wrapping a ClassNotFoundException.
Many thanks to @gotwarlost for demonstrating how to reproduce the problem and diagnosing the cause.

Upgrade to commons-io 2.4

In order to maintain backwards compatibility we need to keep IOUtils the
same as in commons-io 1.4. This code is backwards compatible, however
most of the methods have been deprecated and should instead use the
org.apache.commons.io.IOUtils version instead.

Javadoc for Queue.Task claimed they were compared using .equals(), but in critical cases this was not actually true.
Does not matter for the common case of Job, but when implementing custom tasks this could cause duplicate queue items (and executables).

[FIXED JENKINS-21159] Merged #663.

JENKINS-20618 diagnostics (also in 1.532.2) are on top of jzlib 1.1.3.

also made its display name localizable

Senseless for the state field to be protected when the enum used for its type is private: no subtype could use it anyway.

https://trello.com/c/NvZt4WXu/15-rawhtmlmarkupformatter

Fix autocompletion for items in folders

Revert 33db5445

plugin might erranously put null into the collection, so filter them out

Apache in its default configuration botches this: http://stackoverflow.com/a/9933890/12916