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/proto/src/proto.rs

@@ -216,6 +216,8 @@ messages!(
     (GetImplementationResponse, Background),
     (GetLlmToken, Background),
     (GetLlmTokenResponse, Background),
+    (GetStagedText, Foreground),
+    (GetStagedTextResponse, Foreground),
     (GetUsers, Foreground),
     (Hello, Foreground),
     (IncomingCall, Foreground),
@@ -411,6 +413,7 @@ request_messages!(
     (GetProjectSymbols, GetProjectSymbolsResponse),
     (GetReferences, GetReferencesResponse),
     (GetSignatureHelp, GetSignatureHelpResponse),
+    (GetStagedText, GetStagedTextResponse),
     (GetSupermavenApiKey, GetSupermavenApiKeyResponse),
     (GetTypeDefinition, GetTypeDefinitionResponse),
     (LinkedEditingRange, LinkedEditingRangeResponse),
@@ -525,6 +528,7 @@ entity_messages!(
     GetProjectSymbols,
     GetReferences,
     GetSignatureHelp,
+    GetStagedText,
     GetTypeDefinition,
     InlayHints,
     JoinProject,