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/editor/src/editor.rs

@@ -12060,6 +12060,10 @@ impl Editor {
         self.buffer.read(cx).read(cx).text()
     }
 
+    pub fn is_empty(&self, cx: &App) -> bool {
+        self.buffer.read(cx).read(cx).is_empty()
+    }
+
     pub fn text_option(&self, cx: &App) -> Option<String> {
         let text = self.text(cx);
         let text = text.trim();