Enable collaborating editing of the commit message input inside the git panel (#24130)

https://github.com/user-attachments/assets/200b88b8-249a-4841-97cd-fda8365efd00

Now all users in the collab/ssh session can edit the commit input
collaboratively, observing each others' changes live.

A real `.git/COMMIT_EDITMSG` file is opened, which automatically enables
its syntax highlight, but its original context is never used or saved on
disk — this way we avoid stale commit messages from previous commits
that git places there.

A caveat: previous version put some effort into preserving unfinished
commit messages on repo swtiches, but this version would not do that
— instead, it will be blank on startup, and use whatever
`.git/COMMIT_EDITMSG` contents on repo switch

Release Notes:

- N/A

---------

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

crates/proto/proto/zed.proto

@@ -311,7 +311,8 @@ message Envelope {
 
         Stage stage = 293;
         Unstage unstage = 294;
-        Commit commit = 295; // current max
+        Commit commit = 295;
+        OpenCommitMessageBuffer open_commit_message_buffer = 296; // current max
     }
 
     reserved 87 to 88;
@@ -2655,7 +2656,12 @@ message Commit {
     uint64 project_id = 1;
     uint64 worktree_id = 2;
     uint64 work_directory_id = 3;
-    string message = 4;
-    optional string name = 5;
-    optional string email = 6;
+    optional string name = 4;
+    optional string email = 5;
+}
+
+message OpenCommitMessageBuffer {
+    uint64 project_id = 1;
+    uint64 worktree_id = 2;
+    uint64 work_directory_id = 3;
 }

crates/proto/src/proto.rs

@@ -249,6 +249,7 @@ messages!(
     (OpenBufferForSymbol, Background),
     (OpenBufferForSymbolResponse, Background),
     (OpenBufferResponse, Background),
+    (OpenCommitMessageBuffer, Background),
     (PerformRename, Background),
     (PerformRenameResponse, Background),
     (Ping, Foreground),
@@ -443,6 +444,7 @@ request_messages!(
     (OpenBufferById, OpenBufferResponse),
     (OpenBufferByPath, OpenBufferResponse),
     (OpenBufferForSymbol, OpenBufferForSymbolResponse),
+    (OpenCommitMessageBuffer, OpenBufferResponse),
     (OpenNewBuffer, OpenBufferResponse),
     (PerformRename, PerformRenameResponse),
     (Ping, Ack),
@@ -554,6 +556,7 @@ entity_messages!(
     OpenBufferById,
     OpenBufferByPath,
     OpenBufferForSymbol,
+    OpenCommitMessageBuffer,
     PerformRename,
     PrepareRename,
     RefreshInlayHints,