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

[Robin Westberg]

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

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Commit messages:
 - Fix whitespace problem
 - Post a message when a dependent PR is retargeted

Changes: https://git.openjdk.java.net/skara/pull/1134/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1134&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-941
  Stats: 30 lines in 3 files changed: 27 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/skara/pull/1134.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1134/head:pull/1134

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