remote-curl: add 'get' capability

remote-curl: add 'get' capability

A future change will want a way to download a file over HTTP(S) using
the simplest of download mechanisms. We do not want to assume that the
server on the other side understands anything about the Git protocol but
could be a simple static web server.

Create the new 'get' capability for the remote helpers which advertises
that the 'get' command is avalable. A caller can send a line containing
'get <url> <path>' to download the file at <url> into the file at
<path>.

Reviewed-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>