When discarding a submodule Desktop says it'll end up in Trash but it doesn't

[niik]

Describe the bug

We tell users that discarded changes can be retrieved from Trash but that's not the case for submodules.

Version & OS

v2.5.4-beta2. Os-independent

Steps to reproduce the behavior

1. echo "foo" > app/static/choosealicence.com/README.md
2. Verify that the submodule shows up as a modified entry

3. Attempt to discard

4. Note how the discard dialog says changes can be retrieved from Trash 5. Discard

Expected behavior

The changed files from the submodule gets moved to Trash

Actual behavior

The submodule changes get reset (after #8218 at least) bypassing the Trash.

Additional context

See #3728 which removed our naive implementation of submodule discarding which caused unexpected results. Let's not repeat that exact blunder but rather find a new and exciting mistake to make.