This tells the remote to send us any tags that point to objects that
we are downloading.

This lets us notify the user of what the remote end is doing while we
wait for it to start sending us the packfile.

This allows us to add capabilitites to both at the same time, keeps
them in sync and removes a lot of code.

gitno_buffer now uses a callback to fill its buffer, allowing us to
use the same interface for git and http (which uses callbacks).

The correct way to advertise out capabilities is by appending them to
the first 'want' line, using SP as separator, instead of NUL as the
server does. Inconsistent documentation lead to the use of NUL in
libgit2.

Fix this so we can request much more efficient packs from the
remote which reduces the indexing time considerably.

If it's not available, an error saying so will be returned when trying
to use a https:// URL.

This also unifies a lot of the network code to use git_transport in
many places instead of an socket descriptor.

Clear the error in pkt when we notice that the remote is starting to
send the packfile.

Fix the format string for Windows networking errors.