Clarify regex for git commit --verbose (#1606)

runtime/syntax/git-commit.yaml

@@ -26,7 +26,7 @@ rules:
 
     # Diffs (i.e. git commit --verbose)
     - default:
-        start: "^diff"
+        start: "^diff --git"
         # Diff output puts a space before file contents on each line so this
         # should never match valid diff output and extend highlighting to the
         # end of the file