http-push: use strbuf instead of fwrite_buffer

http-push: use strbuf instead of fwrite_buffer

The http-push code defines an fwrite_buffer function for use
as a curl callback; it just writes to a strbuf. There's no
reason we need to use it ourselves, as we know we have a
strbuf. This lets us format directly into it, rather than
dealing with an extra temporary buffer (which required
manual length computation).

While we're here, let's also remove the literal tabs from
the source in favor of "\t", which is more visually obvious.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>