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>
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