This brings it in line with the rest of the lookup functions.

Git for Windows will handle UNC paths only when in forward-slash
format, eg "//server/path".  When given a UNC path as a remote,
rewrite standard format ("\\server\path") into this ridiculous
format.

 * 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://").

Windows can't handle a path like `/c:/foo`; when turning file:///
URIs into local paths, we must strip the leading slash.

Introduce `cl_assert_equal_oid`

As git_clone now has callbacks to configure the details of the
repository and remote, remove the lower-level functions from the public
API, as they lack some of the logic from git_clone proper.

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

Assert the exact amount of links we expect. While there, check that a
plain git_clone() automatically chooses to link.