RFR: JDK-8261034: improve jcmd GC.class_histogram to support parallel [v6]

[Hamlin Li]

On Sun, 7 Feb 2021 03:29:36 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

> diagnosticCommand.hpp still needs a copyright update.
> 
Thanks for pointing out.
> For heapRegionSet.cpp, sorry but you'll need to file a new CR to update its copyright. Among other things, fixing it here makes backports harder to get right.

I see, revert the related change.

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PR: https://git.openjdk.java.net/jdk/pull/2379