Implement git.untrackedChanges preference

[colin-grant-work]

What it does

Fixes #11253 by implementing the git.untrackedChanges preference.

How to test

1. Open a workspace in a git repository.
2. Modify some files and add some files.
3. Open the SCM view
4. Test the values of the git.untrackedChanges preference:
   • if mixed, untracked and modified should be shown together, as they are on master.
   • if separate, untracked and modified should be shown separately.
   • if hidden, untracked files should not be shown in the SCM view.
5. Test the preference values in both list and tree view modes of the SCM tree.
6. Observe that the Stage All command now ignores untracked files unless git.untrackedFiles is set to mixed, and is disabled if there is nothing to stage.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[vince-fugnitto]

For transparency, I mentioned in an offline comment that it seems the individual toolbar items for the tree nodes in the scm view did not appear when the preference was set to separate:

For comparison, the items are visible in vscode for the same preference value:

[colin-grant-work]

For transparency, I mentioned in an offline comment that it seems the individual toolbar items for the tree nodes in the scm view did not appear when the preference was set to separate:

Good catch. I have pushed a commit that updates the registrations for those actions to include the untracked section.

[vince-fugnitto]

The changes work well for me 👍

I confirmed that all three preference values work well, with different nested structures. Git operations on the nodes also work well and are properly handled by all three preference values.