[Rev 01] RFR: Make it possible to only allow PRs against a select set of target branches

[Robin Westberg]

> Hi all,
> 
> Please review this change that makes it possible to only allow PRs to target a select set of branches.
> 
> Best regards,
> Robin

The pull request has been updated with 1 additional commit.

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Added commits:
 - 7b261d73: Provide a better error message

Changes:
  - all: https://git.openjdk.java.net/skara/pull/377/files
  - new: https://git.openjdk.java.net/skara/pull/377/files/7c3c3098..7b261d73

Webrevs:
 - full: https://webrevs.openjdk.java.net/skara/377/webrev.01
 - incr: https://webrevs.openjdk.java.net/skara/377/webrev.00-01

  Stats: 26 lines in 3 files changed: 20 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/skara/pull/377.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/377/head:pull/377

PR: https://git.openjdk.java.net/skara/pull/377