RFR: 340: Infer master for repos in the same org for merge style PRs

[Robin Westberg]

Hi all,

Please review this change that allows a shorter form when merging the master branch from a repo in the same org.

Best regards,
Robin

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Commit messages:
 - If a short branch name is invalid, assume it is the name of a sibling repo

Changes: https://git.openjdk.java.net/skara/pull/554/files
 Webrev: https://webrevs.openjdk.java.net/skara/554/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-340
  Stats: 74 lines in 3 files changed: 71 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/skara/pull/554.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/554/head:pull/554

PR: https://git.openjdk.java.net/skara/pull/554