CI: don't fetch the whole git history if it's not required

[bayandin]

Problem

We do use actions/checkout with fetch-depth: 0 when it's not required

Summary of changes

• Remove unneeded fetch-depth: 0
• Add a comment if fetch-depth: 0 is required

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[koivunej]

I see 24s to checkout, sounds excellent, down from 3min.

[github-actions]

4994 tests run: 4830 passed, 0 failed, 164 skipped (full report)

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Flaky tests (5)

Postgres 17

• test_pageserver_compaction_smoke: release-arm64
• test_location_conf_churn[1]: release-arm64
• test_pg_regress[None]: debug-x86-64
• test_scrubber_physical_gc[4]: release-arm64

Postgres 16

• test_slots_and_branching: release-x86-64

Code coverage* (full report)

• functions: 32.0% (7413 of 23182 functions)
• lines: 49.8% (59572 of 119717 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
7fa02cd at 2024-09-17T13:06:05.296Z :recycle:}