git: do not validate submodules of fresh checkouts

[osiewicz]

Fixes #14603

What does this PR try to resolve?

As is, we unconditionally validate freshness of the submodules of a checkout, even though we could assume that a fresh checkout has to have up-to-date submodules as well.

How should we test and review this PR?

N/A

Additional information

N/A

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[epage]

We likely should update _corrupted_checkout test to have a submodule to show that we recovery gracefully on interruption,

[epage]

Could you update the commits to be how you want them reviewed and merged?

For example, I could see this being

• Test commit
• Refactor for checkout guard
• Improve performance by adjusting the scope of the checkout guard

[epage]

FYI ecb6398 was what I was asking for in splitting out a refactor commit. It makes it easier to follow what is intended with the actual performance fix.