This PR updates the styling of the tree branches in the current call
section of the collab panel:

- Fixed the gap between the notes and chat items.
- We now overdraw the tree branch on the notes item so it fills the gap.
- The tree branch lines are now thinner.

<img width="231" alt="Screenshot 2024-01-08 at 12 35 35 PM"
src="https://github.com/zed-industries/zed/assets/1486634/844c9e18-c2de-4163-9384-41d1577e23a0">

Note: I think the signature for `render_tree_branch` could use some
improvement. I don't like having the multiple `bool` parameters, but any
change I could think of was going to be a bit more invasive than I
wanted to take on in this PR.

Release Notes:

- Improved the style of the tree branches in current call details in the
collab panel.