Fix "refusing to merge unrelated histories" (#3626)

## Motivation and Context
Fixes the last [failed backport workflow
run](https://github.com/smithy-lang/smithy-rs/actions/runs/8971718857/job/24638066793)

## Description
The error `refusing to merge unrelated histories` was caused by the same
reason described in prior PRs (#3479, #3480). We need to tell git
unshallow histories so that it knows they are actually related.

## Testing
Ran the failed workflow from this branch and successfully generated a
backport PR3625.

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