When a list of refspecs is passed to fetch (what git would consider
refspec passed on the command-line), we not only need to perform the
updates described in that refspec, but also update the remote-tracking
branch of the fetched remote heads according to the remote's configured
refspecs.

These "fetches" are not however to be written to FETCH_HEAD as they
would be duplicate data, and it's not what the user asked for.

The configured/base fetch refspecs need to be taken into account in
order to implement opportunistic remote-tracking branch updates. DWIM
them and store them in the struct, but don't do anything with them yet.

This option make it easy to ignore anything about the server we're
connecting to, which is bad security practice. This was necessary as we
didn't use to expose detailed information about the certificate, but now
that we do, we should get rid of this.

If the user wants to ignore everything, they can still provide a
callback which ignores all the information passed.

 * Move the transport registration mechanisms into a new header under
   'sys/' because this is advanced stuff.
 * Remove the 'priority' argument from the registration as it adds
   unnecessary complexity.  (Since transports cannot decline to operate,
   only the highest priority transport is ever executed.)  Users who
   require per-priority transports can implement that in their custom
   transport themselves.
 * Simplify registration further by taking a scheme (eg "http") instead
   of a prefix (eg "http://").

git_remote_set_transport now takes a transport factory rather than a transport
git_clone_options now allows the caller to specify a remote creation callback

This allows us to add e.g. "HEAD" as a refspec when none are given
without overwriting the user's data.

Move this one as well, letting us have a single way of setting the
callbacks for the remote, and removing fields from the clone options.

This is a significant reorganization of the diff code to break it
into a set of more clearly distinct files and to document the new
organization.  Hopefully this will make the diff code easier to
understand and to extend.

This adds a new `git_diff_driver` object that looks of diff driver
information from the attributes and the config so that things like
function content in diff headers can be provided.  The full driver
spec is not implemented in the commit - this is focused on the
reorganization of the code and putting the driver hooks in place.

This also removes a few #includes from src/repository.h that were
overbroad, but as a result required extra #includes in a variety
of places since including src/repository.h no longer results in
pulling in the whole world.

It used to be separate as an attempt to make the querying easier, but
it didn't work out that way, so put all the data together.

Add git_refspec_string() as well to get the original string, which is
now stored alongside the independent parts.