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/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,