[SPARK-35082][INFRA] Use permissive and squshed merge when syncing to the latest branch in GitHub Actions testing

[HyukjinKwon]

What changes were proposed in this pull request?

There is an issue when syncing to the Apache master branch, see also #32168:

From https://github.com/yaooqinn/spark
 * branch                  SPARK-35044 -> FETCH_HEAD
fatal: Not possible to fast-forward, aborting.
Error: Process completed with exit code 128.

This is because we use --ff-only option so it assumes that the fork is always based on the latest master branch.
We should make it less strict.

This PR proposes to use the same command when we merge PRs:

spark/dev/merge_spark_pr.py

Line 127 in c8f56eb

run_cmd(['git', 'merge', pr_branch_name, '--squash'])

Why are the changes needed?

To unblock PR testing broken.

Does this PR introduce any user-facing change?

No, dev-only.

How was this patch tested?

Locally tested

Co-authored-by: Kent Yao yaooqinn@hotmail.com
Co-authored-by: Yikun Jiang yikunkero@gmail.com

Closes #32168

[github-actions]

Test build #750453913 for PR 32183 at commit 4b22125.

[HyukjinKwon]

@dongjoon-hyun, @wangyum, @yaooqinn, @gengliangwang, @viirya , @Yikun can you take a quick look please? I would like to merge as soon as possible and see how it gose if you guys don't mind.

[HyukjinKwon]

Oh, just saw the PR #32182. Thanks @Yikun !

[dongjoon-hyun]

Ya, it's really active community. :)