Instead of using ifdefs to run the tests, use them to set when we expect
to support a particular scheme and always have the tests in the code.

It does the same as git_remote_supported_url() but has a name which
implies we'd check the URL for correctness while we're simply looking at
the scheme and looking it up in our lists.

While here, fix up the tests so we check all the combination of what's
supported.

We used to assume a refspec would only have an asterisk in the middle of
their respective pattern. This has not been a valid assumption for some
time now with git.

Instead of assuming where the asterisk is going to be, change the logic
to treat each pattern as having two halves with a replacement bit in the
middle, where the asterisk is.

netops: error out on url without a path

In order to connect to a remote server, we need to provide a path to the
repository we're interested in. Consider the lack of path in the url an
error.

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

This was a bad idea. Don't free except in the free function.