RFR: 731: Do not include target hash in cached PR metadata

[Robin Westberg]

Avoid using the target branch hash in check metadata calculation, as the pull request update cache will hide such
updates anyway.

Best regards,
Robin

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Commit messages:
 - Drop target hash from metadata calculation

Changes: https://git.openjdk.java.net/skara/pull/850/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=850&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-731
  Stats: 13 lines in 3 files changed: 3 ins; 4 del; 6 mod
  Patch: https://git.openjdk.java.net/skara/pull/850.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/850/head:pull/850

PR: https://git.openjdk.java.net/skara/pull/850