fix: support submodules when commit signing

[peter-evans]

Fixes #3353

[github-actions]

Full test suite slash command (repository admin only)

/test repository=peter-evans/create-pull-request ref=fix-commit-signing-for-submodules build=true

/test repository=peter-evans/create-pull-request ref=fix-commit-signing-for-submodules build=true sign-commits=true

[peter-evans]

/test repository=peter-evans/create-pull-request ref=fix-commit-signing-for-submodules build=true sign-commits=true

Command run output

[peter-evans]

/test repository=peter-evans/create-pull-request ref=fix-commit-signing-for-submodules

Command run output