Restructure git diff state management to allow viewing buffers with different diff bases (#21258)

This is a pure refactor of our Git diff state management. Buffers are no
longer are associated with one single diff (the unstaged changes).
Instead, there is an explicit project API for retrieving a buffer's
unstaged changes, and the `Editor` view layer is responsible for
choosing what diff to associate with a buffer.

The reason for this change is that we'll soon want to add multiple "git
diff views" to Zed, one of which will show the *uncommitted* changes for
a buffer. But that view will need to co-exist with other views of the
same buffer, which may want to show the unstaged changes.

### Todo

* [x] Get git gutter and git hunks working with new structure
* [x] Update editor tests to use new APIs
* [x] Update buffer tests
* [x] Restructure remoting/collab protocol
* [x] Update assertions about staged text in
`random_project_collaboration_tests`
* [x] Move buffer tests for git diff management to a new spot, using the
new APIs

Release Notes:

- N/A

---------

Co-authored-by: Richard <richard@zed.dev>
Co-authored-by: Cole <cole@zed.dev>
Co-authored-by: Conrad <conrad@zed.dev>

crates/collab/src/rpc.rs

@@ -309,6 +309,7 @@ impl Server {
             .add_request_handler(forward_read_only_project_request::<proto::ResolveInlayHint>)
             .add_request_handler(forward_read_only_project_request::<proto::OpenBufferByPath>)
             .add_request_handler(forward_read_only_project_request::<proto::GitBranches>)
+            .add_request_handler(forward_read_only_project_request::<proto::GetStagedText>)
             .add_request_handler(forward_mutating_project_request::<proto::UpdateGitBranch>)
             .add_request_handler(forward_mutating_project_request::<proto::GetCompletions>)
             .add_request_handler(