RFR: 2304: /backport jdk23 should provide a more helpful message

[Zhao Song]

This patch is trying to improve the bot's replies of backport command.

1. If the user issued `/backport jdkX`, the bot would check if there is a `jdkX` branch in current repo, if so, the bot would tell the user that to target a backport to `jdkX` branch in current repo, he should issue `/backport :jdkX`.

2. When user specified branch name in the command but the branch doesn't exist, the bot will list 10 branch names in the target repo, the branch names are just sorted in reverse alphabetical order, so we can't guarantee they are the most relevant ones.

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Commit messages:
 - Fix tests
 - SKARA-2304

Changes: https://git.openjdk.org/skara/pull/1667/files
  Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1667&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2304
  Stats: 46 lines in 2 files changed: 42 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/skara/pull/1667.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1667/head:pull/1667

PR: https://git.openjdk.org/skara/pull/1667