cat-file: use oidset check-and-insert

cat-file: use oidset check-and-insert

We don't need to check if the oidset has our object before
we insert it; that's done as part of the insertion. We can
just rely on the return value from oidset_insert(), which
saves one hash lookup per object.

This measurable speedup is tiny and within the run-to-run
noise, but the result is simpler to read, too.

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