RFR: 941: Warn about required merge after dependent PR integration

[Robin Westberg]

On Thu, 22 Apr 2021 13:33:55 GMT, Robin Westberg <rwestberg at openjdk.org> wrote:

> When a dependent PR has been integrated, post an instructional message on how to merge these changes.

@stefank As the original issue reporter, does this look reasonable to you?

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PR: https://git.openjdk.java.net/skara/pull/1134