i18n: git-branch "(no branch)" message

Gettextize the "(no branch)" message that's shown by "git branch" when
you're in a detached HEAD.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3203-branch-output.sh

@@ -72,7 +72,7 @@ cat >expect <<'EOF'
   branch-two
   master
 EOF
-test_expect_success 'git branch shows detached HEAD properly' '
+test_expect_success C_LOCALE_OUTPUT 'git branch shows detached HEAD properly' '
 	git checkout HEAD^0 &&
 	git branch >actual &&
 	test_cmp expect actual