RFR: cli: make git-pr-create detect backports

[Erik Helin]

Hi all,

please review this patch that makes `git pr create` uses the correct title for "backport style" pull requests. If the branch only contains a single commit that matches a "backport style" pull request title, then `git pr create` should use that commit's commit message title for the pull request title (even if the branch refers to an existing JBS issue).

Testing:
- [x] Manual testing on Linux x64

Thanks,
Erik

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Commit messages:
 - cli: make git-pr-create detect backports

Changes: https://git.openjdk.java.net/skara/pull/1039/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1039&range=00
  Stats: 15 lines in 1 file changed: 6 ins; 2 del; 7 mod
  Patch: https://git.openjdk.java.net/skara/pull/1039.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1039/head:pull/1039

PR: https://git.openjdk.java.net/skara/pull/1039