Ensure untracked files are listed by git in core themes

[westonruter]

Because /src/wp-content/theme is ignored in .gitignore, any new files added to core themes will not be listed when doing git status, and any such files will not be added with doing git add without --force, giving the error message: “The following paths are ignored by one of your .gitignore files”.

Following up on #27207.

[westonruter]

I tried to commit but I got an SVN pre-commit hook error:

$ svn ci .gitignore
Sending        .gitignore
Transmitting file data .svn: E165001: Commit failed (details follow):
svn: E165001: Failed to start '/home/svn/repos/wp-develop/hooks/pre-commit' hook
svn: E165001: Your commit message was left in a temporary file:
svn: E165001:    '.../vvv/www/wordpress-develop/svn-commit.tmp'

[westonruter]

In 39362:

Git: Prevent untracked files from being ignored by git in bundled themes.

See #27207.
Fixes #38779.