RFR: Move PR target ref resolution to the utilities class

[Robin Westberg]

The target ref resolution is better done locally to avoid races and also unnecessary API calls when only the ref name is used.

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Commit messages:
 - Move PR target ref resolution to the utilities class

Changes: https://git.openjdk.java.net/skara/pull/940/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=940&range=00
  Stats: 92 lines in 15 files changed: 7 ins; 47 del; 38 mod
  Patch: https://git.openjdk.java.net/skara/pull/940.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/940/head:pull/940

PR: https://git.openjdk.java.net/skara/pull/940