git: Amend (#28187)

Adds git amend support.

- [x] Turn existing commit button into split button
- [x] Clean up + Handle shortcuts/focus cases
- [x] Test remote

Release Notes:

- Added git amend support.

---------

Co-authored-by: Cole Miller <cole@zed.dev>

crates/proto/proto/git.proto

@@ -292,6 +292,11 @@ message Commit {
     optional string name = 4;
     optional string email = 5;
     string message = 6;
+    optional CommitOptions options = 7;
+
+    message CommitOptions {
+        bool amend = 1;
+    }
 }
 
 message OpenCommitMessageBuffer {