session: fix auto commit variable change effect on transaction commit (#36631)

[ti-srebot]

cherry-pick #36631 to release-6.1
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# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/36755

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---

What problem does this PR solve?

Issue Number: close #36581

Problem Summary:

What is changed and how it works?

When changing the value of the session variable auto-commit, the implicit commit for ongoing transactions should happen only when it's changed from off to on.

Check List

Tests

• Unit test

Side effects

Documentation

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

Fix the auto-commit mode change related transaction commit behaviours.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• you06
• zyguan

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[ti-srebot]

/run-all-tests

[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/6f67940385dc6acd822c7bcf06ac611a9835807f

[zyguan]

LGTM

[cfzjywxk]

/approve

[cfzjywxk]

/merge

[ti-chi-bot]

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[cfzjywxk]

/merge

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cfzjywxk, you06, zyguan

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-06-30 05:01:20.367122478 +0000 UTC m=+61763.741700330: ☑️ agreed by cfzjywxk.
• 2023-06-30 05:50:14.409660325 +0000 UTC m=+64697.784238176: ☑️ agreed by zyguan.

[cfzjywxk]

/merge

[ti-chi-bot]

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[cfzjywxk]

Why couldn't it be merged..

[Ivy-YinSu]

/retest

[tiprow]

@Ivy-YinSu: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

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/retest

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[Ivy-YinSu]

/test mysql-test

[tiprow]

@Ivy-YinSu: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

In response to this:

/test mysql-test

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