t3200: add test case for 'branch -m'

Signed-off-by: Stefan Naewe <stefan.naewe@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3200-branch.sh

@@ -74,6 +74,11 @@ test_expect_success \
         git branch -d l/m &&
         git branch l'
 
+test_expect_success \
+    'git branch -m dumps usage' \
+       'test_expect_code 129 git branch -m 2>err &&
+	grep "[Uu]sage: git branch" err'
+
 test_expect_success \
     'git branch -m m m/m should work' \
        'git branch -l m &&