snapshot: Report match in --diff if there are no changes

The user experience of 'sudo port snapshot --diff $id --all' just
printing nothing is not great. Improve this by explicitly printing
a success message if there are no changes.

(cherry picked from commit 35e8fc4)

src/macports1.0/snapshot.tcl

@@ -141,6 +141,7 @@ namespace eval snapshot {
                         }
                     }
                 }
+
                 if {[llength $diff(removed)] > 0} {
                     append note "The following ports are in the snapshot but not installed:\n"
                     foreach removed_port [lsort -ascii -index 0 $diff(removed)] {
@@ -169,6 +170,10 @@ namespace eval snapshot {
                     }
                 }
 
+                if {[llength $diff(added)] == 0 && [llength $diff(removed)] == 0 && [llength $diff(changed)] == 0} {
+                    append note "The current state and the specified snapshot match.\n"
+                }
+
                 ui_msg [string trimright $note "\n"]
                 return 0
             }