Fix syntax highlighting of git commit messages (#26988)

- Load syntax colors into commit message editors
- Fix name mismatches that were preventing the git commit grammar and
language config from being matched up

Release Notes:

- Fixed git commit messages not being syntax-highlighted

crates/languages/src/gitcommit/config.toml

@@ -1,5 +1,5 @@
 name = "Git Commit"
-grammar = "git_commit"
+grammar = "gitcommit"
 path_suffixes = [
   "TAG_EDITMSG",
   "MERGE_MSG",

crates/languages/src/gitcommit/highlights.scm

@@ -4,6 +4,7 @@
 (commit) @constant
 (item) @markup.link.url
 (header) @tag
+(comment) @comment
 
 (change kind: "new file" @diff.plus)
 (change kind: "deleted" @diff.minus)
@@ -15,4 +16,3 @@
   value: (trailer_value) @string)
 
 [":" "=" "->" (scissors)] @punctuation.delimiter
-(comment) @comment