Follow up to https://github.com/zed-industries/zed/pull/54406.
Closes https://github.com/zed-industries/zed/issues/54782.

In the PR linked above, we made the label that's displayed in the
project button be always the repository name. However, if you're opening
up a subfolder of that repository as the main-root of a given
project/workspace, we wouldn't display that anymore. This PR fixes that
by _still_ displaying the repo name, but the subfolder, too, in the
titlebar:

<img width="500" alt="Screenshot 2026-04-24 at 4  52@2x"
src="https://github.com/user-attachments/assets/04c2c61d-874d-4694-88c9-06bad9677e49"
/>

Release Notes:

- Fixed a bug where we don't display subfolders of a repository in the
title bar's project button.