fix(git): do not use `+=` in sh script

Fixes #12251

plugins/git-commit/git-commit.plugin.zsh

@@ -46,7 +46,7 @@ case $1 in
     shift 1
     ;;
   * )
-    _message+=" $1"
+    _message="${_message} $1"
     shift 1
     ;;
 esac