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/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 },
+]

crates/languages/src/gitcommit/highlights.scm

@@ -0,0 +1,18 @@
+(subject) @markup.heading
+(path) @string.special.path
+(branch) @string.special.symbol
+(commit) @constant
+(item) @markup.link.url
+(header) @tag
+
+(change kind: "new file" @diff.plus)
+(change kind: "deleted" @diff.minus)
+(change kind: "modified" @diff.delta)
+(change kind: "renamed" @diff.delta.moved)
+
+(trailer
+  key: (trailer_key) @variable.other.member
+  value: (trailer_value) @string)
+
+[":" "=" "->" (scissors)] @punctuation.delimiter
+(comment) @comment

crates/languages/src/gitcommit/injections.scm

@@ -0,0 +1,5 @@
+((scissors) @content
+ (#set! "language" "diff"))
+
+((rebase_command) @content
+ (#set! "language" "git_rebase"))