cat-file: move "usage" variable to cmd_cat_file()

cat-file: move "usage" variable to cmd_cat_file()

There's no benefit to defining this at a distance, and it makes the
code harder to read as you've got to scroll up to see the usage that
corresponds to the options.

In subsequent commits I'll make use of usage_msg_opt(), which will be
quite noisy if I have to use the long "cat_file_usage" variable,
there's no other command being defined in this file, so let's rename
it to just "usage".

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