RFR: Allow '+' to be in branch names in merge PR titles [v3]

[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.

Jorn Vernee has updated the pull request incrementally with one additional commit since the last revision:

  of -> in

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Changes:
  - all: https://git.openjdk.java.net/skara/pull/970/files
  - new: https://git.openjdk.java.net/skara/pull/970/files/5ea7abcf..48242706

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=skara&pr=970&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=skara&pr=970&range=01-02

  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 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