ls-refs: reuse buffer when sending refs

ls-refs: reuse buffer when sending refs

In the initial reference advertisement, the Git server will first
announce all of its references to the client. The logic is handled in
`send_ref()`, which will allocate a new buffer for each refline it is
about to send. This is quite wasteful: instead of allocating a new
buffer each time, we can just reuse a buffer.

Improve this by passing in a buffer via the `ls_refs_data` struct which
is then reused on each reference.

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