show-branch: free() allocated "head" before return

Stop leaking the "head" variable, which we've been leaking since it
was originally added in [1], and in its current form since [2]

1. ed378ec7e8 (Make ref resolution saner, 2006-09-11)
2. d9e557a320 (show-branch: store resolved head in heap buffer,
   2017-02-14).

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

builtin/show-branch.c

@@ -956,5 +956,6 @@ int cmd_show_branch(int ac, const char **av, const char *prefix)
 		if (shown_merge_point && --extra < 0)
 			break;
 	}
+	free(head);
 	return 0;
 }