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>

Cargo.lock

@@ -5254,7 +5254,6 @@ dependencies = [
  "picker",
  "postage",
  "project",
- "rpc",
  "schemars",
  "serde",
  "serde_derive",
@@ -7008,6 +7007,7 @@ dependencies = [
  "tree-sitter-cpp",
  "tree-sitter-css",
  "tree-sitter-diff",
+ "tree-sitter-gitcommit",
  "tree-sitter-go",
  "tree-sitter-gomod",
  "tree-sitter-gowork",
@@ -13944,6 +13944,15 @@ dependencies = [
  "tree-sitter-language",
 ]
 
+[[package]]
+name = "tree-sitter-gitcommit"
+version = "0.0.1"
+source = "git+https://github.com/zed-industries/tree-sitter-git-commit?rev=88309716a69dd13ab83443721ba6e0b491d37ee9#88309716a69dd13ab83443721ba6e0b491d37ee9"
+dependencies = [
+ "cc",
+ "tree-sitter-language",
+]
+
 [[package]]
 name = "tree-sitter-go"
 version = "0.23.4"