RFR: 956: Skara bot is continually trying to process the same PR command

[Robin Westberg]

Invalidate related GET cache entries when performing a non-GET request, to ensure that we for example always retrieve an updated list of comments after posting a new one.

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Commit messages:
 - Invalidated related GET cache entries when performing a non-GET request

Changes: https://git.openjdk.java.net/skara/pull/1129/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1129&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-956
  Stats: 40 lines in 2 files changed: 40 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/1129.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1129/head:pull/1129

PR: https://git.openjdk.java.net/skara/pull/1129