Rework shared commit editors (#24274)

Rework of https://github.com/zed-industries/zed/pull/24130
Uses
1033c0b57e
`COMMIT_EDITMSG` language-related definitions (thanks @d1y )

Instead of using real `.git/COMMIT_EDITMSG` file, create a buffer
without FS representation, stored in the `Repository` and shared the
regular way via the `BufferStore`.
Adds a knowledge of what `Git Commit` language is, and uses it in the
buffers which are rendered in the git panel.


Release Notes:

- N/A

---------

Co-authored-by: Conrad Irwin <conrad@zed.dev>
Co-authored-by: d1y <chenhonzhou@gmail.com>
Co-authored-by: Smit <smit@zed.dev>

crates/proto/proto/zed.proto

@@ -2693,6 +2693,7 @@ message Commit {
     uint64 work_directory_id = 3;
     optional string name = 4;
     optional string email = 5;
+    string message = 6;
 }
 
 message OpenCommitMessageBuffer {