builtin/cat-file: rename variable that tracks usage

builtin/cat-file: rename variable that tracks usage

The usage strings for git-cat-file(1) that we pass to `parse_options()`
and `usage_msg_optf()` are stored in a variable called `usage`. This
variable shadows the declaration of `usage()`, which we'll want to use
in a subsequent commit.

Rename the variable to `builtin_catfile_usage`, which is in line with
how the variable is typically called in other builtins.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>