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/git/src/repository.rs

@@ -1,6 +1,6 @@
 use crate::status::FileStatus;
-use crate::GitHostingProviderRegistry;
 use crate::{blame::Blame, status::GitStatus};
+use crate::{GitHostingProviderRegistry, COMMIT_MESSAGE};
 use anyhow::{anyhow, Context as _, Result};
 use collections::{HashMap, HashSet};
 use git2::BranchType;
@@ -62,7 +62,7 @@ pub trait GitRepository: Send + Sync {
     /// If any of the paths were previously staged but do not exist in HEAD, they will be removed from the index.
     fn unstage_paths(&self, paths: &[RepoPath]) -> Result<()>;
 
-    fn commit(&self, message: &str, name_and_email: Option<(&str, &str)>) -> Result<()>;
+    fn commit(&self, name_and_email: Option<(&str, &str)>) -> Result<()>;
 }
 
 impl std::fmt::Debug for dyn GitRepository {
@@ -283,14 +283,22 @@ impl GitRepository for RealGitRepository {
         Ok(())
     }
 
-    fn commit(&self, message: &str, name_and_email: Option<(&str, &str)>) -> Result<()> {
+    fn commit(&self, name_and_email: Option<(&str, &str)>) -> Result<()> {
         let working_directory = self
             .repository
             .lock()
             .workdir()
             .context("failed to read git work directory")?
             .to_path_buf();
-        let mut args = vec!["commit", "--quiet", "-m", message];
+        let commit_file = self.dot_git_dir().join(*COMMIT_MESSAGE);
+        let commit_file_path = commit_file.to_string_lossy();
+        let mut args = vec![
+            "commit",
+            "--quiet",
+            "-F",
+            commit_file_path.as_ref(),
+            "--cleanup=strip",
+        ];
         let author = name_and_email.map(|(name, email)| format!("{name} <{email}>"));
         if let Some(author) = author.as_deref() {
             args.push("--author");
@@ -450,7 +458,7 @@ impl GitRepository for FakeGitRepository {
         unimplemented!()
     }
 
-    fn commit(&self, _message: &str, _name_and_email: Option<(&str, &str)>) -> Result<()> {
+    fn commit(&self, _name_and_email: Option<(&str, &str)>) -> Result<()> {
         unimplemented!()
     }
 }