RFR: pr: use per-user branch for backports

[Erik Helin]

Hi all,

please review this patch that makes the backport branches created by the bot per user.

Thanks,
Erik

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Commit messages:
 - pr: use per-user branch for backports

Changes: https://git.openjdk.java.net/skara/pull/1125/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1125&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/1125.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1125/head:pull/1125

PR: https://git.openjdk.java.net/skara/pull/1125