RFR: git-pr: support git pr create --draft

[Erik Helin]

Hi all,

please review this patch adds support for `--draft` switch to `git pr create`.

Testing:
- Manual testing of `git pr create`

Thanks,
Erik

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Commits:
 - c667d5a6: Support git pr create --draft

Changes: https://git.openjdk.java.net/skara/pull/454/files
 Webrev: https://webrevs.openjdk.java.net/skara/454/webrev.00
  Stats: 6 lines in 1 file changed: 5 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/454.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/454/head:pull/454

PR: https://git.openjdk.java.net/skara/pull/454