RFR: 2252: Backport command branch names not unique enough

[Zhao Song]

To avoid the name clash of backport branch in the bot's fork, we could add `targetBranchName` to the backport branch name.

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Commit messages:
 - SKARA-2252

Changes: https://git.openjdk.org/skara/pull/1651/files
  Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1651&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2252
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/skara/pull/1651.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1651/head:pull/1651

PR: https://git.openjdk.org/skara/pull/1651