RFR: Move PR target ref resolution to the utilities class [v2]

[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.

Robin Westberg has updated the pull request incrementally with one additional commit since the last revision:

  Restore fetch on missing ref

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Changes:
  - all: https://git.openjdk.java.net/skara/pull/940/files
  - new: https://git.openjdk.java.net/skara/pull/940/files/56bcf097..d3dbae26

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=skara&pr=940&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=skara&pr=940&range=00-01

  Stats: 8 lines in 1 file changed: 6 ins; 0 del; 2 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