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.

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.

There is no reason why we need to use a callback here. A string array
fits better with the usage, as this is not an event and we don't need
anything from the user.

When there is a reference in the target namespace, we should overwrite
it. Instead it gets a different name under the current code.

Inside `git_remote_load`, the calls to `get_optional_config` use
`giterr_clear` to unset any errors that are set due to missing config
keys. If neither a fetch nor a push url config was found for a remote,
we should set an error again.

If the remote supports the symref protocol extension, then we return
that, otherwise we guess with git's rules.