RFR: pr: relaxed matching of backport PR titles

[Erik Helin]

Hi all,

please review this patch that makes the matching for "backport" style pull requests a bit more relaxed with regards to whitespace.

Testing:
- [x] Added three new unit tests

Thanks,
Erik

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Commit messages:
 - pr: relaxed matching of backport PR titles

Changes: https://git.openjdk.java.net/skara/pull/955/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=955&range=00
  Stats: 178 lines in 2 files changed: 177 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/955.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/955/head:pull/955

PR: https://git.openjdk.java.net/skara/pull/955