Rework shared commit editors (#24274)

Rework of https://github.com/zed-industries/zed/pull/24130
Uses
https://github.com/d1y/git_firefly/tree/1033c0b57ec88a002cb68efc64c8d9bf5c212e30
`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/worktree/src/worktree.rs

@@ -199,7 +199,7 @@ pub struct RepositoryEntry {
     ///     - my_sub_folder_1/project_root/changed_file_1
     ///     - my_sub_folder_2/changed_file_2
     pub(crate) statuses_by_path: SumTree<StatusEntry>,
-    pub work_directory_id: ProjectEntryId,
+    work_directory_id: ProjectEntryId,
     pub work_directory: WorkDirectory,
     pub(crate) branch: Option<Arc<str>>,
 }