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>