2304: /backport jdk23 should provide a more helpful message

[zhaosongzs]

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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Progress

• [ ] Change must be properly reviewed (1 review required, with at least 1 Reviewer)
• [x] Change must not contain extraneous whitespace

Issue

• SKARA-2304: /backport jdk23 should provide a more helpful message (Bug - P3)

Reviewing

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