RFR: Allow '+' to be in branch names in merge PR titles

[Jorn Vernee]

Allow '+' to be in branch names in merge PR titles. I've also sharpened the error message to include the exact pattern the bot checks for.

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Commit messages:
 - Add test, make error message clearer
 - Allow '+' characters in branch names of Merge PRs

Changes: https://git.openjdk.java.net/skara/pull/970/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=970&range=00
  Stats: 52 lines in 2 files changed: 48 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/skara/pull/970.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/970/head:pull/970

PR: https://git.openjdk.java.net/skara/pull/970