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 &&