fetch-pack: Use a strbuf to compose the want list
This change is being offered as a refactoring to make later
commits in the smart HTTP series easier.
By changing the enabled capabilities to be formatted in a strbuf
it is easier to add a new capability to the set of supported
capabilities.
By formatting the want portion of the request into a strbuf and
writing it as a whole block we can later decide to hold onto
the req_buf (instead of releasing it) to recycle in stateless
communications.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>