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>

crates/languages/src/gitcommit/config.toml

@@ -0,0 +1,18 @@
+name = "Git Commit"
+grammar = "git_commit"
+path_suffixes = [
+  "TAG_EDITMSG",
+  "MERGE_MSG",
+  "COMMIT_EDITMSG",
+  "NOTES_EDITMSG",
+  "EDIT_DESCRIPTION",
+]
+line_comments = ["#"]
+brackets = [
+  { start = "(", end = ")", close = true, newline = false },
+  { start = "`", end = "`", close = true, newline = false },
+  { start = "\"", end = "\"", close = true, newline = false },
+  { start = "'", end = "'", close = true, newline = false },
+  { start = "{", end = "}", close = true, newline = false },
+  { start = "[", end = "]", close = true, newline = false },
+]