fetch-pack: buffer object-format with other args

fetch-pack: buffer object-format with other args

In send_fetch_request(), "object-format" is written directly to the file
descriptor, as opposed to the other arguments, which are buffered.
Buffer "object-format" as well. "object-format" must be buffered; in
particular, it must appear after "command=fetch" in the request.

This divergence was introduced in 4b831208bb ("fetch-pack: parse and
advertise the object-format capability", 2020-05-27), perhaps as an
oversight (the surrounding code at the point of this commit has already
been using a request buffer.)

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>