RFR: pr: only create backport branch once

[Erik Helin]

Hi all,

please review this patch that ensures that the `/backport` commit command will only create the backport commit (and corresponding branch) once. There is no need to try to do this multiple times, the cherry picking will result in the same result every time. If a user for some reasons issues the `/backport` command twice on a commit then this will now work.

Thanks,
Erik

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Commit messages:
 - pr: only create backport branch once

Changes: https://git.openjdk.java.net/skara/pull/1107/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1107&range=00
  Stats: 129 lines in 2 files changed: 79 ins; 18 del; 32 mod
  Patch: https://git.openjdk.java.net/skara/pull/1107.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1107/head:pull/1107

PR: https://git.openjdk.java.net/skara/pull/1107