How to check the diff between git commited files and modified or created files?

git diff shows the difference between old commited files and new files. Here is an example.

$ git diff
diff --git a/logger.lisp b/logger.lisp
index dda9a4c..2fa0def 100644
--- a/logger.lisp
+++ b/logger.lisp
@@ -6,10 +6,17 @@
 (provide 'my-logger)
 
 (defpackage "MY-LOGGER"
-  (:nicknames "LOGGER"))
+  (:nicknames "LOGGER")
+  (:use "COMMON-LISP" "CCL" "COMMON-LISP-USER")
+  (:export "LOG-DEBUG"
+          "LOG-INFO"
+          "LOG-NOTICE"
+          "LOG-WARNING"
+          "LOG-ERROR"
+          "LOG-ALERT"
+          "LOG-CRITICAL"))