RFR: git-pr-create: support --backport option

[Erik Helin]

Hi all,

please review this patch that adds the `--backport=HASH` option to `git pr create` to make it a bit easier to create a
"backport style" pull request from the CLI. A backporter can now only has to run:

$ git checkout -b backport-8123456
$ git cherry-pick <HASH>
$ git pr create --backport=<HASH>

Thanks,
Erik

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Commit messages:
 - git-pr-create: support --backport option

Changes: https://git.openjdk.java.net/skara/pull/871/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=871&range=00
  Stats: 30 lines in 1 file changed: 18 ins; 3 del; 9 mod
  Patch: https://git.openjdk.java.net/skara/pull/871.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/871/head:pull/871

PR: https://git.openjdk.java.net/skara/pull/871