cat-file: rename cmdmode to transform_mode

cat-file: rename cmdmode to transform_mode

In the next patch, we will add an enum on the batch_options struct that
indicates which type of batch operation will be used: --batch,
--batch-check and the soon to be  --batch-command that will read
commands from stdin. --batch-command mode might get confused with
the cmdmode flag.

There is value in renaming cmdmode in any case. cmdmode refers to how
the result output of the blob will be transformed, either according to
--filter or --textconv. So transform_mode is a more descriptive name
for the flag.

Rename cmdmode to transform_mode in cat-file.c

Signed-off-by: John Cai <johncai86@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>