Use concat instead of push around listFolders

[aeisenberg]

This avoids stack overflows when using the spread operator on directories that have many, many children.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Confirm the readme has been updated if necessary.
• Confirm the changelog has been updated if necessary.

[henrymercer]

Looks good. While we're here, perhaps it's worth auditing the rest of the spread operators in the codebase to make sure these aren't likely to cause a stack overflow.

[aeisenberg]

I did a quick look and did not find any other likely dangerous locations.