File completion for `:vnew` and `:tabnew` doesn't seem to work

Describe the bug
The tabsuggestion for the :vnew and :tabnew commands produces incomplete and incorrect results

To Reproduce
Steps to reproduce the behavior:

1. Open any project
2. Type <<escape>> :tabnew <<tab>>
3. Observe that many (but not all?) files in the current directory are not suggested. Observe that files that are not in the current directory but subfolders get suggested (and opening them fails)

Expected behavior

All files and folders in the current directory get suggested. Files and folders that are not in the current directory do not get suggested.

Screenshots
If applicable, add screenshots to help explain your problem.
If remapping-related, please attach log output: https://github.com/VSCodeVim/Vim#debugging-remappings.

Environment (please complete the following information):

• Extension (VsCodeVim) version: 1.16.0

• VSCode version: 1.47.3

• OS: Fedora Linux 31 amd64

Additional context
Add any other context about the problem here.

Start by reproducing file completion with :vnew and :tabnew in a project, checking both current-directory files and nested paths. Trace the TypeScript completion handling for these commands; done means all files and folders in the current directory are suggested, while nested paths are excluded and suggested entries open successfully.