Rework shared commit editors (#24274)

Rework of https://github.com/zed-industries/zed/pull/24130
Uses
1033c0b57e
`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.toml

@@ -522,6 +522,7 @@ tree-sitter-cpp = "0.23"
 tree-sitter-css = "0.23"
 tree-sitter-elixir = "0.3"
 tree-sitter-embedded-template = "0.23.0"
+tree-sitter-gitcommit = {git  = "https://github.com/zed-industries/tree-sitter-git-commit", rev = "88309716a69dd13ab83443721ba6e0b491d37ee9"}
 tree-sitter-go = "0.23"
 tree-sitter-go-mod = { git = "https://github.com/camdencheek/tree-sitter-go-mod", rev = "6efb59652d30e0e9cd5f3b3a669afd6f1a926d3c", package = "tree-sitter-gomod" }
 tree-sitter-gowork = { git = "https://github.com/zed-industries/tree-sitter-go-work", rev = "acb0617bf7f4fda02c6217676cc64acb89536dc7" }