Tars plugin has redundant merge base operations

[Mini256]

Bug Report

• I have searched the issues of this repository and believe that this is not a duplicate.

What is expected?

Tars do not perform unnecessary merge-base operations.

What is actually happening?

case 1: pingcap/tidb#25792

In this case, tars was triggered almost twice at the same time. It may be that the push event and the timed task triggered tars almost at the same time, resulting in repeated merge base operations.

• pingcap/tidb@4795564
• pingcap/tidb@759caa0

case 2：pingcap/tidb-operator#4171

In this case, PR is modified based on the latest base, and three commits are submitted in this PR, but the tars plugin that is triggered regularly is considered the base of PR is out-of-date.

case 3: tikv/tikv#10746

In this case, the robot will reply to two comments after /merge. Such three comments will trigger tars at the same time, and there will be a race condition problem.

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