cat-file: split batch "buf" into two variables

cat-file: split batch "buf" into two variables

We use the "buf" strbuf for two things: to read incoming
lines, and as a scratch space for test-expanding the
user-provided format. Let's split this into two variables
with descriptive names, which makes their purpose and
lifetime more clear.

It will also help in a future patch when we start using the
"output" buffer for more expansions.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>