remote-curl: make refs_url a strbuf

remote-curl: make refs_url a strbuf

In the discover_refs function, we use a strbuf named
"buffer" for multiple purposes. First we build the info/refs
URL in it, and then detach that to a bare pointer. Then, we
use the same strbuf to store the result of fetching the
refs.

Let's instead keep a separate refs_url strbuf. This is less
confusing, as the "buffer" strbuf is now used for only one
thing.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Jonathan Nieder <redacted>